When a calligrapher writes on paper, their hand moves with an intuitive understanding of proportions, rhythm, and structural rules. In the digital realm, individual glyphs lack this inherent awareness. This is where the OpenType Specification—developed jointly by Microsoft and Adobe—steps in as the underlying logic engine that transforms static shapes into fluent, context-aware Arabic typography.

Unlike Latin scripts, Arabic typography is a continuous, connected system. It relies heavily on two primary lookup tables within the font binary: the Glyph Substitution table (GSUB) and the Glyph Positioning table (GPOS). Together, they dictate how characters interact dynamically based on their context.

1. Positional Forms: The Foundation of Connectivity

Due to the cursive nature of the Arabic alphabet, a glyph changes its visual representation depending on its position within a word. These layout features are hardcoded essentials:

■ isol (Isolated): The stand-alone form of a letter when unconnected on either side.
■ init (Initial): Triggered at the start of a word segment to prepare for forward connection.
■ medi (Medial): Activated in the middle of a sequence to join on both sides.
■ fina (Final): Applied at the end of a word segment to seal the stroke connection.

2. Ligatures: Managing Typographic Space

Ligatures in Arabic are not merely decorative flourishes; they are crucial engineering tools used to solve visual collisions and manage spatial balance:

■ rlig (Required Ligatures): Mandatory substitutions essential for orthographic correctness, such as the Lam-Alef combination.
■ liga (Standard Ligatures): Default typographic joins that enhance reading flow, especially in traditional Naskh styles.
■ dlig (Discretionary Ligatures): Optional artistic joinings enabled by the user for specialized calligraphic effects.
■ clig (Contextual Ligatures): Smart substitutions that inspect surrounding glyphs to prevent awkward ascender/descender collisions.

3. Diacritic Engineering and Vertical Positioning

Accurate placement of vocalization marks (Harakat) on Arabic letters used to be a technical hurdle. Modern OpenType positioning rules resolved this cleanly:

■ ccmp (Glyph Composition): Combines multiple stacked diacritics into a single logical unit for cleaner rendering.
■ mark (Mark-to-Base): Defines exact X/Y anchor coordinates to attach marks to base letters, defined using tools like Glyphs.
■ mkmk (Mark-to-Mark): Prevents diacritic collision when multiple marks stack vertically on top of one another.

4. Alternates & Stylistic Sets

These features grant designers the ability to embed versatile typographic personalities within a single font file:

■ salt & aalt: Primary interfaces for accessing all design variants of a given character.
■ swsh (Swash): Activates elegant terminal extensions and swooping tails on word endings.
■ jalt (Justification Alternates): Provides elongated character variants (Kashida) for precise line-justification.
■ locl (Localized Forms): Automatically swaps glyph shapes according to regional conventions, such as Maghrebi dot positions or Urdu language preferences.
■ ss01 - ss20 (Stylistic Sets): Global switches that alter the entire aesthetic tone of the typeface with a single click.

5. Spacing, Kerning, and Cursive Attachment

■ curs (Cursive Attachment): Manages smooth step-like vertical and horizontal connections required in slanted scripts like Nastaliq and Ruq'ah.
■ kern (Kerning): Fine-tunes horizontal distances between non-connecting character pairs to eliminate uncomfortable visual gaps.

6. Numerical Systems

■ pnum (Proportional Figures): Digits with variable widths designed to blend seamlessly within running body text.
■ tnum (Tabular Figures): Monospaced digits engineered for financial alignment and structured data tables.
■ frac (Fractions): Automatically converts slash-separated numbers into clean typographic fractions.