Joining Forms

Arabic script is a cursive writing system; i.e, letters can join to their neighboring letters. In this perspective, 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 forms for U+0645 ARABIC LETTER MEEM (م).

For each Arabic letter, based on the joining behavior of its neighbors, one of its shapes is used in writing. demonstrates how letters join to form a word.

There are different categories of characters based on their joining behavior, but most of the Arabic letters are either dual joining or right joining. Dual joining characters can join from both sides. Like the character in image 1, these types of characters have all the four shapes mentioned above. Right joining characters only join to their previous (right-side) character. These characters only have isolated and final shapes, for they don’t join to their next character.

Almost all the non-alphabetical characters are non-joining. The few exceptions will be discussed in this document.

Please refer to The Unicode Standard Version 8.0, Section 9.2, for full explanation of Arabic cursive joining.

Joining XXX

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?

1. 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 to more typical fonts:
   

   

2. 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.

3. 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.

4. 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.

5. 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 .

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.