Gap Analysis

This document is pointed to by a separate document, Arabic & Persian Gap Analysis, which describes gaps in support for Arabic and Persian on the Web, and prioritises and describes the impact of those gaps on the user.

Wherever an unsupported feature is indentified through the gap analysis process, the requirements for that feature need to be documented. This document is where those requirements are described.

This document should contain no reference to a particular technology. For example, it should not say "CSS does/doesn't do such and such", and it should not describe how a technology, such as CSS, should implement the requirements. It is technology agnostic, so that it will be evergreen, and it simply describes how the script works. The gap analysis document is the appropriate place for all kinds of technology-specific information.

Other Related Resources

The document International text layout and typography index (known informally as the text layout index) points to this document and others, and provides a central location for developers and implementers to find information related to various scripts.

The W3C also maintains a tracking system that has links to github issues in W3C repositories. There are separate links for (a) requests from developers to the user community for information about how scripts/languages work, (b) issues raised against a spec, and (c) browser bugs. For example, you can find out what information developers are currently seeking, and the resulting list can also be filtered by script.

Standard Arabic Language

Standard Arabic—a.k.a. Modern Standard Arabic or Literary Arabic—is the standardized and literary variety of Arabic used in writing and in most formal speech in countries of Northern Africa and West Asia. Regional and classical dialects of Arabic may differ in layout and text details and are not covered by this document.

However, there are some major differences in common practices between the Western Arab regions—that is North-West Africa—and Eastern Arab regions—which is North-East Africa and West Asia. For example, the numeral digits used in the two regions and their formatting are vastly different. Although, there's no clear line between the Eastern and Western Arab regions.

Persian Language

Persian—a.k.a. Modern Persian—is the standardized and literary variety of the official languages used in Iran and Afghanistan. The dialect of Persian in Iran is also called Western Persian, and is locally known as Farsi. The dialect of Persian in Afghanistan is also known as Eastern Persian, and is locally known as Dari.

Tajik—a.k.a Tajiki or Tajiki Persian—is the Persian language as used in Tajikistan. It is written in the Cyrillic script, therefore, is not covered by this document.

Joining Forms

Every Arabic letter has one, two, or four different joining forms, which allow the letter to join to its neighbors, if applicable. These four forms are:

• Isolated form, used when the letter does not join to any of the surrounding letters;
• Initial form, used when the letter is joining only to its next (left-hand side) letter;
• Medial form, used when the letter is joining on both sides, and
• Final form, used when the letter is joined only to its previous (right-hand side) letter.

shows samples of all four joining forms for U+0645 ARABIC LETTER MEEM (م).

We define the following two groups of joining forms:

• Join-to-left forms: either Initial form or Medial form of a letter, which joins to the left-hand side (next) letter. Other forms are called non-join-to-left.
• Join-to-right forms: either Medial form or Final form of a letter, which joins to the right-hand side (previous) letter. Other forms are called non-join-to-right.

Joining Categories

There are different categories of letters based on their joining behavior:

• Dual-joining letters: can join from both sides, like the letter in , and has all the four shapes mentioned above.
• Right-joining letters: can only join to their previous (right-hand side) letter, and therefore, only have isolated and final shapes. shows samples of both forms for U+0631 ARABIC LETTER REH (ر).
• Non-joining letters: cannot join to any surrounding letter, and therefore can only take the isolated form. shows a sample of U+0621 ARABIC LETTER HAMZAH (ء) in its only possible form.

Joining Rules

There are core rules on how letters join to each other in the Arabic script, which stay valid regardless of the medium (hand-writing, typewriter, movable-type, digital, etc):

1. Letters of each word join together whenever possible, implicitly.
2. In some languages, like Persian and Urdu, there are words—mostly, but not limited to, compound words—that require explicit breaks in the joining of letters, although joining would otherwise be possible.
3. In certain cases, a letter can be in a join-to-left form without actually connecting to anything on the left, whether there’s any letter or not. This is often seen in list counters, abbreviations, and other cases where letters do not have a word context, or are taken out of their original word context.
4. In rare cases of words splitting where letters are joined, first letter of the second half will be in a join-to-right form without any previous letter. This behavior is limited to special cases like blanking specific letters of a word, line breaks in a paragraph, and word breaks across poetry verses. No standalone word can have any letters in join-to-right form without joining on the right-hand side.

demonstrates how letters join (per Joining Rule 1) to form a word.

Joining Control

Arabic letters are represented in their intended joining forms in hand-writing, typewriters, and old (deprecated) digital encodings of the script. In Unicode, letters are encoded semantically—meaning without any information about their joining form—and therefore there’s need for a mechanism for controlling of the joining behavior of the letters.

In Unicode, by default, neighbor Arabic letters join together if and only if both letters are able to join towards the other.

Disjoining Enforcement

As noted in Joining Rule 2, sometimes two Arabic letters sit next to each other (in one word) which would normally join together, but should not. In Unicode, for such a case, a special character should be used to enforce disjoining of these letters. This character is called U+200C ZERO WIDTH NON-JOINER, or ZWNJ for short.

Joining Enforcement

Similarly, as noted in Joining Rule 4, sometimes an Arabic letter needs to take a joining form when it would not happen normally. For example, some abbreviation methods use Initial Form of letters, when possible, for every letter in the abbreviation. Again, in Unicode, a special character should be used to enforce joining on this letter. This character is called U+200D ZERO WIDTH JOINER, or ZWJ for short.

Besides ZWJ, there’s another special Unicode character, U+0640 ARABIC TATWEEL, which enforces joining behavior (join causing) on letters next to it. But, in contrast to ZWJ, TATWEEL has a glyph shape, looking like a hyphen and usually as wide as the SPACE glyph, which connects to the letters on the main joining line (a.k.a. base-line). So, using TATWEEL would give a similar Joining Enforcement behavior, but has a side effect of wider length for the letter, which is not always desired. That’s why it’s highly recommended to only use ZWJ for joining control.

Joining-Disjoining Enforcement

Two enforcement methods mentioned above can be combined together to form a Joining-Disjoining Enforcement method, that enables Joining Rule 3 for cases when there’s a dual-joining/right-joining letter after a join-to-left letter, which should not be joined to its previous letter.

Joining Segments

A sequence of letters that join together are called a Joining Segment. Regardless of language, joining segments have no direct relationship to syllables.

Two types of joining segments exist: closed and open.

Non-Joining Characters

Arabic Letters, two Joining Control Characters (ZWNJ and ZWJ), and TATWEEL are the only characters used in the Arabic writing system with joining behavior.

Arabic diacritics, other Unicode non-spacing marks, and most Unicode format control characters are considered transparent in joining behavior.

All other Unicode characters in Arabic script (as well as Latin and many other major scripts) are non-joining and do not take any joining forms other than Isolated.

For more the details on Arabic Cursive Joining algorithm, please refer to chapter Middle East-I — Modern and Liturgical Scripts of The Unicode Standard. [[!UNICODE]]

Arabic Style and Calligraphy

Arabic styling and writing has its origins in Islamic art and civilization, and was widely used to decorate mosques and palaces, as well as to create beautiful manuscripts and books, and especially to copy the Korʼan. Arabic script is cursive, making it viable to support different geometric shapes overlapping and composition. Words can be written in a very condensed form as well as stretched into elongated shapes, and the scribes and artists of Islam labored with passion to take advantage of all these possibilities.

From the beginning of Arabic calligraphy, two tendencies or two types of styles can be seen emerging: writing for the decoration of mosques and sculptures, which was complex and highly decorative, and writing styles reserved for writing the Korʼan, which were easier to use and more readable.

Writing styles then evolved under the influences of cultural diversity, leading to regional calligraphic schools and styles (Kufi in Iraq, Farsi and Taʻlīq in Persia, or Diwani in Turkey). Additional differences arose depending on the purpose of writing, such as the copying and dissemination of the Korʼan.

In general we group under the generic term Naskh (copy/inscription) the scripts which are meant for reading at smaller sizes and are suitable for books and texts to be read, e.g. the Korʼan, and as Kufic the highly stylized font styles used for ornamentation and more styled writings. Nevertheless, the rich evolution of the Arabic script led to the distinctive enumeration of a number of additional named styles.

Similarly, two other generic terms are used to classify styles : Mabsut (wa Mustaqīm) is a form of style that is elongated and straight angled, [which dominated the copies of Korʼan in eighth and ninth centuries], and Muqawwar (wa Mudawwar) is a form of style that is curved and rounded.

Different Writing Styles

Basics and principles of Arabic writing were defined by Ibn Moqlah (886-940 Higra), who defined six styles of writing: Kufi, Thuluth, Naskh, Ruqʻa, Taʻlīq and Diwani.

Kufi (كوفي)

One of the oldest and best known Arabic scripts. It is characterized by its decorative and pronounced geometric forms, well adapted for architectural designs. The style grew with the beginning of Islam to satisfy a need for Muslims to codify the Korʼan.

Thuluth (ثلث)

(The third.) Recognizable by the fact that the letters and words are highly interleaved in its complex form. May be the most difficult style to write (requiring a significant amount of skill), both in terms of its letters and in terms of its structure and composition.

Naskh (نسخ)

One of the clearest styles of all, with clearly distinguished letters which facilitate reading and pronunciation. Can be written at small sizes (traditionally using pens made of reeds and ink), which suits the production of longer texts used for boards and books intended for the general population, especially the Korʼan.

Ruqʻa (رقعة‎)

A handwritten style still commonly used in Arabic countries, and recognisable by its bold-like letters written above the writing line. Designed to be used for education, for everyday writing and adopted in the offices (Diwan) of the Ottoman Empire. One of it's feature is that calligraphers have kept it and did not derived variations from it.

Taʻlīq (تعليق)

Taʻlīq (hanging) is a beautiful script characterized by the precision and stretch of its letters, its clarity, and its lack of complexity. Designed for Persian language, until replaced by Nastaʻlīq.

Diwani (ديواني)

Used by the Ottman court (Diwan) to write official documents. Some variations of it are still in use today (e.g. hand written documents by some religious officials).

We can add other font styles to this list, such as the following :

Nastaʻlīq (نستعلیق)

Persian version of Taʻlīq, derived from Naskh and Taʻlīq and developed in the 8th and 9th centuries. It is like a Taʻlīq but easier to write and read. Shekasteh Nastaʻlīq (literally means "broken Nastaʻlīq") is also another derivation of those two, developed in the 15th century.

Maghribi (مغربي)

Used in the past in the western islamic world (Andalusia), and still now in North Africa. Used for writing the Korʼan as well as other scientific, legal and religious manuscripts. Rabat, a mabsut version of it, is widely used in some official printings in Morocco.

Arabic Script and Typography

Arabic script has some characteristics that are challenging for typographers and font designers. Examples bellow show some characteristics worth to be considered carefully. How could typography, which came late to the Arabic world, then follow the tradition of the many authors and artists who manually shaped the Arabic writing over decades? even in it's simplest Naskh style?

Multi-level baselines

Letters may join through a finely inclined line

or two, square-ended lines

Multilevel baselines don't occur in all fonts. The above examples use the Arabic Typesetting font. Compare those examples to more typical fonts:

Multi-context joining

Rendering of letters depends not only on their place in the word (initial, medial, final) but also on their neighboring letters, i.e. the letter they join with. Each letter has a different appearance in each combination.

Fonts don't always comply with or respect this kind of tuning. To do so, fonts need many glyphs in order to adapt to each context. In more modern typefaces some of these connections are implemented by ligatures, but ligatures can't capture or cover all joining behavior.

In the two left most words, the initial noon differs in that one raises a kind of stroke. This property of raising a stroke is common for a number of letters (beh, teh, noon, theh) which are taller than their connected letters in order to be distinguished in some contexts, such as vs. , or to resolve ambiguity. See also the section about teeth letters below.

Words as groups of letters

A word shape is not (only) a "horizontal" connections of letters, but of groups of letters (syntagmes).

Example two words in some nice Naskh font.

Groups of letters are colored blue or red

To compare with the same words in more usual font:

Can't really say letter groups. Rather a "horizontal sequence of letters of almost same width".

Group combinations cannot be covered by general or usual ligatures.

Vertical joining

Groups of letters may also "join" vertically (top down) instead of right to left. And not all fonts permit this.

	vs.
Joining happens almost vertical		Joining happens horizontal

Once again, some fonts try standard ligatures, but this is not ligature. This is rather (good) writing practice/style.

One should note that all this characteristics has not only an aesthetic side, but also play a role in justification. It is at the discretion of (hand writing) authors to chose the best kind of joining to suit the desired line width. Should then be a general rule on that. But to achieve such justification would require sophisticated algorithms.

The so called teeth letters.

Letters having uniform medial shape, align in a kind of teeth.

Even in the teeth context letter shape may vary. It's not the same letters (in red) which raise the stroke in the two figures.

Fonts

Arabic script counts 26 letters, and mostly 19 basic shapes. Since letters change according to their position in the word, Arabic set of glyph may range to more than one hundred shapes. If one count possible ligatures, and different combination of joining forms (see above), the number of glyph can increase further. Not sure that typeface design can accommodate all needs, even though some present typefaces can run hundred of shapes.

Early typefaces, some still in use today, were designed with some facilities. Designer of those differs in their simplification hypothesis. For example, one of the first approach is to use "type writer style", that is a same glyph for different positions in a word. This is the case for initial and medial shape for most of the letters (example here). It is generally the browser default font for Arabic script. A more unifying approach is the use of a single and detached glyph for each letter without joining (todo example here). Other approach were used resulting in more or less visually practical fonts.

Nowadays, there is a large choice of fonts, and one can choose the font that best suits to one's typographical desire. However, one may also wishes to take into account some non typographical considerations like: (TBD later on)

• Accessibility (readability and visibility) ...
• The kind of device with small screen (for example, larger loop and teeth height, small descenders etc...), although fonts actually appear better on smartphones
• Font style for titles and banners and alike (small number of words), may differ from the style for content text (long text).
• Shapes and proportions (the size issue) in mixed texts
• Some fonts might give another opportunity for line justification than the one based on word spacing (See section 4.2.4 Ligatures).
• etc...

Joining and Intra-Word Spaces

The only spaces inside Arabic words are created near characters that are not dual-joining. When adjusing intra-word spaces (i.e. the space inside the words) only these spaces can be adjusted. Moving two joined characters closer to or further from each other creates undesirable results.

Transparency

Arabic fonts achieve joining by overlapping letters. A left-joining letter extends out of its bounding box from the left side and a right-joining letter extends out of its bounding box from the right side. Making each letter transparent can expose these overlapping joinings, which should be avoided. Joining the paths of the joined letter into a single shape can remove the overlappings and create the good results.

Text border

When adding text border, simply adding a border to each letter shape fails to produce the proper result for the Arabic script. A joined letter should not be separated from its joined neighbors by adding border. Like transparency, a way to avoid this is to unify glyph paths into a single big path for all the letters that are joined and add border around that path.

Styling individual letters

For educational, technical, or even aesthetic reasons, users might want to apply a specific style to a single letter (or a few letters) in a word. For example, is the logo of the largest telecommunications provider in Oman.

This should not break the letter’s joining with its neighbors, as shown in .

Preferred Terminology

Before entering this section in we need to introduce few preferred terminological conventions for disambiguation and simplicity.

• European Numerals are 0, 1, 2, 3, 4, 5, 6, 7, 8, 9. They are also referred to as Western Arabic Numerals or simply as Arabic Numerals. Although these are terminologically correct terms, to avoid confusions we will refrain from using these phrases to refer to these numerals. European Numerals or ASCII numerals are used instead.
• Arabic-Indic Numerals are ٠, ١, ٢, ٣, ٤, ٥, ٦, ٧, ٨, ٩.
• Eastern Arabic-Indic Numerals are ۰, ۱, ۲, ۳, ۴, ۵, ۶, ۷, ۸, ۹.
• Extended Arabic-Indic Numerals same as Eastern Arabic-Indic Numerals.
• Western Arabic Numerals, or Arabic Numerals same as European Numerals.
• Eastern Arabic Numerals is used to refer to both Arabic-Indic Numerals and Eastern Arabic-Indic Numerals. Should be avoided due to ambiguity.
• Indic Numerals should be avoided to refer to either of Arabic-Indic or Eastern Arabic-Indic numerals.
• Digit, Numeral digit, and Numeral are used as synonyms.

Families of Numerals

There are three families of numerals used with languages using the Arabic Script: the European Numerals, the Arabic-Indic Numerals, and Eastern Arabic-Indic Numerals. The following table, based on a similar table in [[W3-ARAB-MATH]] exhibits these three families. Those three numerals which have different shapes between Arabic-Indic Numerals and Eastern Arabic-Indic Numerals are highlighted.

In all Arabic numeral systems, numbers are written with the lowest significant digits to the right and the highest digits to the left (a.k.a. left to right), though Arabic script is written from right to left.

An important fact to note here is the bidirectional category of these numbers.

• European digits and Eastern Arabic-Indic digits are of category EN (European Number).
• Arabic-Indic digits are of category AN (Arabic Number), differently from their counterpart just above.

The difference in bidirectional category between Arabic-Indic digits and Eastern Arabic-Indic digits is due to the difference in the behavior desired in Arabic vs. Persian.

As a consequence, the following sentences, having the similar content, result in a very different ordering in a right-to-left context:

Formatting of Numerals

European numerals are used with "," (U+002C COMMA) and "." (U+002E FULL STOP) as decimal separator and thousands separator, respectively, or vice versa; depending on the region.

• 1.234,5 in Western regions.
• 1,234.5 in Eastern regions.
• Sometimes, U+2009 THIN SPACE or U+202F NARROW NO-BREAK SPACE may also be used as thousands separator.

Arabic-Indic numerals use two specific separators:

• "٫" (U+066B ARABIC DECIMAL SEPARATOR)
• "٬" (U+066C ARABIC THOUSANDS SEPARATOR)
• Example: ١٬٢٣٤٫٥ .

Arabic number in other uses

Numbers do not always appear alone, and may appear alongside other characters like financial symbols, fraction signs, decimal and/or thousands signs (excluding math expressions here). Also there are Arabic-specific signs such as ؉ [U+0609 ARABIC-INDIC PER MILLE SIGN] and ٪ [U+066A ARABIC PERCENT SIGN], as well as the Arabic decimal " ٫ " (U+66B) and thousands  " ٬ " (U+066C) separators, mentioned earlier. These are mostly used with Arabic-Indic digits.

Numerals can also come separated by or mixed with space or other signs. Example are phone numbers +12 34 56 78 89, cars licence plate like 123 د‎ 4,  quantities 37.5°,   ٥٠ كلم (50km) etc.

Particular attention is needed here. Firstly, numbers have a weak directionality with regards to the Bidi algorithm. For example, alongside a number, certain otherwise neutral characters, such as negative/positive sign, currency or degree symbols, are likely to be treated as part of the number rather than a neutral.

Secondly, the placement of the accompanying signs and symbols may depend on the region: generally Middle East (or anglophone) vs. Western (or francophone) regions. This is not to mention punctuation signs.

• The percent sign is placed on the left after the number (ie. ٪١٢ not *١٢٪), and without a space (*٪ ١٢). With European numbers, % [U+0025 PERCENT SIGN] is sometimes used, and can be placed either on the right or the left of the digits (eg. 12% or %12).
• Arabic decimal and thousand separators obey the same rule as for European numbers (١٬٢٣٤٫٥٦). European signs are used with European numbers (1.234,56 or 1,234.56).
• Money or currency signs when they come alongside a number, are placed at the left and treated as part of the number rather than a neutral (€12.3 or €١٢٫٣).
• On the other hand, degree signs are placed at the right of the number (37.5° C, ٩٩٫٥° F) or (37.5° م, ۹۹٫۵° ف).
• When indicating quantity, there should a separating space (12 كغ, ٤٥ مم).
• The same rule applies when using range of values. (12-15 كغ, but 12٪-15٪ كغ).
• / [U+002F SOLIDUS] is used for fractions or ratio notation. Fractions are noted for one-half, say, 2/1 or ٢/١, mostly in RTL mode. There is no standard approach, however, and some region/author may write 1/2 for European digits.

  May be use image for examples.

• Notation with Solidus sign " / " are used in:
  
  • Notation such as speed "50 km/h"
    كم/س ‎ 50
  • Notation such as "Page / Volume", for example a reference to a page in a volume
    ١٢٣/١ for volume ١, page ١٢٣
  • or reference to Quran "Verse / Surah",
    ١٣/٩٠ for Surah 90, Verse 13
  • Mark or rank in a classification
    20/15 for rank 15 over 20
  • Dates (2017/06/24, ٢٠١٧/٠٦/٢٤)

    Look for evidence for some examples

Other issues

How to know that a sign (space, comma ...) is a separator or a sign within a number? +12 34 56 78 90 is a phone number or a sequence of digits? Which may be inverted in RTL. A tip is to use a syntax like 12.34.56.78.90 or 12-34-56-78-90 for phones.

Maybe mention the Decimal Separator Key Symbol " ⎖ " (U+2396), used with keyboards (resembles an apostrophe)

Characters that cannot end or start a line

Hyphenation

Adjusting Inter-Word Spaces

This is the same mechanism widely used when justifying Latin scripts, where the width of the spaces between the words can be increased or decreased to change the width of the line.

A minimum width is defined for how much the space can be shrunk, because putting the words too close to each other creates aesthetic and legibility problems.

Stretching the space too wide is also undesirable, but is utilized as a last resort when it is not possible to use other solutions to make fully justified paragraphs. In some applications a maximum width for the inter-word space is defined as a soft limit (compared to minimum width which is a hard limit). Reaching the maximum width makes the software to try to use other solutions for justification. If no other solution could yield the required result, the software would fall back to inter-word spacing and stretch the space past the maximum width.

Depending solely on this mechanism for aligning lines in a justified paragraph can lead to unpleasant results, such as rivers (multiple stretched spaces appearing vertically close to each other and forming a white gap inside the paragraph) and uneven distribution of color in the paragraph. Hence, typographers generally use other mechanisms as well to minimize the effect of adjusting inter-word spaces.

Adjusting Intra-Word Spaces

This solution alters the space between letters of each word to change the width of the text. Like adjusting inter-word spaces, this is used for Latin scripts as well, but using it for Arabic script involves considerations specific to Arabic. As noted in Joining and Intra-Word Spaces, the principal consideration is that gaps between characters only exist for those letters that join only to the right, such as dal and reh . Adjustment of intra-word space is not relevant where one letter is joined to its neighbors.

Depending on the writing style and the typeface in use, different amounts of alteration to the intra-word space is acceptable for Arabic. Some writing styles allow more liberal adjustments to the closeness of the letter groups, while others can only accept small adjustments in this regard. In any case, much smaller adjustments can be used for intra-word spacing in comparison for inter-word spacing, which naturally is wider and tolerate bigger adjustments.

Alternative Shapes

In addition to the four joining forms (isolated, initial, medial, and final), each Arabic letter can come with different shapes while preserving its joining form. For instance, a typeface or writing style can offer two or more shapes for the final form of a single letter.

These variant shapes usually have variant widths and hence can be used to adjust the width of the line.

An advantage of using alternative letter shapes when justifying paragraphs is that it does not involve modifying default properties of the typeface (width of space or other characters). Instead, it is using shapes that are part of the typeface and are in harmony with other shapes in the lines.

But excessive use of alternative shapes, such as using multiple very wide alternatives close to each other, can create unnatural results.

It is not possible to justify paragraphs using only alternative letter shapes, because these shapes have predefined widths. For example, if a line should get 25 points wider, it is impossible to achieve that by using alternative letter shapes that are, say, 10 or 20 or 30 points wider than the default shapes. But these shapes can make the lines closer to measure, thus reducing the usage of other mechanisms.

Ligatures

Some Arabic fonts, following the writing styles that use special shapes when joining certain letters, provide a rich number of ligatures. These ligatures can be used in paragraph justification, since they usually reduce the widths of the words.

But existence of the ligatures in a font does not mean that they can be used freely. A font may provide some of its ligatures for creating an artistic style, which would be unsuitable for texts requiring optimum legibility.

For that reason, the user should be able to select which sets of ligatures can be used for justification. Fonts can offer predefined sets of ligatures to simplify this process.

Kashida

Kashida refers to extending the horizontal connection between joined letters.

This is a feature deeply related with the cursive nature of Arabic script. Kashida is an interesting tool for paragraph justification. It is more flexible than alternative letter shapes and ligatures, because it is not restricted to a limited number of predefined widths. At the same time, it has relatively less effect on the text color than spacing.

But a proper implementation of kashida involves a number of limitations and considerations.

Excessive use of kashida or applying very long kashidas results in uneven color. Also, horizontal or vertical proximity of numerous kashida creates an unnatural color.

Kashida is not always straight. Some fonts may require curvilinear kashidas, which require more advanced implementations.

Typographers can have preferred places for applying kashidas. In other words, instead of applying kashida between every joined pair of letters, they want it at certain joins.

Another preference is avoiding multiple kashidas in a single word.

Tatweel

Tatweel is a dual-joining character that can be inserted between two joined letters to widen their connection. In The Unicode Standard, tatweel is represented as U+0640 ARABIC TATWEEL (ـ).

Tatweel extends letter connections in a fashion similar to kashida, but in a much more limited way. It is a character that has to be in the text or inserted like other characters. It has a predefined width, like any other character.

Yet it is much simpler to implement, since it acts like normal Arabic characters and does not require special treatment. For this reason, it can be considered useful specially in constrained implementations like fixed-width environments.

Combination of the Mechanisms

Each of the above six mechanisms have their own limitations and side effects. Utilizing only one of them for justifying paragraphs can create undesirable results. Multiple mechanisms can be used at the same time to work around their limitations and minimize their side effects.

Since Arabic provides various mechanisms that can be used for justification, an advanced implementation that supports all or most of the above features can produce exemplary justifications. More limited applications can combine what is available.

Preferences for each mechanisms can depend on the document and text and the preference of the typographers and users. Implementations can enable users to prioritize and control the mechanisms mentioned above.

Vertical text

In situations where short runs of text run vertically, for example on book spines or in table headers, Arabic text is rotated to run along the line. It may be rotated so that the tops of the letters face to the left (read the text from top to bottom) or to the right (read the text from bottom to top).

 

The flow of text, top-down vs. bottom-up, may depend on regions or authors. The left case in is a typically francophone style for book spines, whereas the right case is an anglophone style.

Arabic embedded in vertically orientated text

When Arabic is embedded in body text that is set vertically, such as CJK or Mongolian text, it is also normally rotated so as to run along the line. Typically, the Arabic text will be read from bottom to top of the line.

When the Arabic text spans more than one line, the text is wrapped in the same way as it would be in horizontal text, ie. the first part of the Arabic text is kept on the first line, and subsequent parts of the Arabic text appear on subsequent lines. Therefore, in the case of Mongolian, where lines are read left-to-right, the Arabic text lines are also read left-to-right, whereas in Chinese or Japanese, where vertical lines are normally read right-to-left, the Arabic text lines are also read right-to-left.

Upright vertical Arabic text

There are attested cases of Arabic text arranged vertically with the letters upright, for example in signboards for cinemas or theatres. It is not clear, at this point whether this a standard approach for Arabic text, or just an unusual layout that mimics Western typographic approaches.

The following should be noted in .

1. Letters flow from top to bottom (as with Latin script).
2. Isolated form of letters seems more appropriate here than the joined form.
3. Diacritic marks, if any, must be applied to letters and not appear on separate lines.

We need to establish whether this is a standard approach or just an oddity. The items in the list above are important to note, however, we need to check whether isolated forms are always used, and the direction is always top to bottom when upright letters are used.