The writing system a character belongs to (e.g., Latin, Cyrillic, Han). Unicode 16.0 defines 168 scripts; the Script property is key for security and mixed-script detection.

What is Unicode Bidirectional Algorithm (UBA)?

Algorithm determining display order of characters in mixed-direction text (e.g., English + Arabic), using character bidi categories and explicit directional overrides.

What is RTL (Right-to-Left)?

Text directionality where characters flow from right to left. Used by Arabic, Hebrew, Thaana, and other scripts; requires the Bidirectional Algorithm for proper display.

Joining Type — Unicode Glossary

Q: What is Joining Type?

Unicode property controlling how Arabic and Syriac characters connect to adjacent characters. Types include Right_Joining, Left_Joining, Dual_Joining, and Non_Joining.

What is Joining Type?

Joining Type is a Unicode character property that describes how a character participates in the cursive joining behavior of Arabic-script (and other scripts that use connected cursive forms, such as Syriac, N'Ko, and Mongolian). It is fundamental to correct text shaping for Arabic and related scripts, because Arabic letters change their visual form depending on their position in a word — specifically, whether they connect to the letters before and after them.

The Unicode standard defines the Joining_Type property with six values, stored in ArabicShaping.txt in the Unicode Character Database.

The Six Joining Types

Value	Abbreviation	Meaning	Example
Right_Joining	R	Connects to the preceding character only (right side)	ا (Alef, U+0627)
Left_Joining	L	Connects to the following character only (left side)	Rare; some extensions
Dual_Joining	D	Connects to both preceding and following characters	ب (Ba, U+0628)
Non_Joining	U	Does not connect to either neighbor	Space, Latin letters
Join_Causing	C	Causes joining but may not be visible	Zero Width Joiner (U+200D)
Transparent	T	Ignored for joining purposes; acts as if absent	Combining marks, some diacritics

How Arabic Letters Change Shape

An Arabic letter's joining type, combined with its position in a joining sequence, determines which of up to four contextual glyph forms (allographs) the font renders:

Isolated form: The letter appears alone or between non-joiners
Initial form: The letter begins a joining sequence (connects to the right, toward the next letter)
Medial form: The letter is in the middle of a sequence (connects on both sides)
Final form: The letter ends a joining sequence (connects to the left, toward the previous letter)

For example, the Arabic letter Ba (ب, U+0628, Dual_Joining) appears as:

Context	Form	Appearance
Standalone	Isolated	ب
Word-initial	Initial	بـ
Word-medial	Medial	ـبـ
Word-final	Final	ـب

This shaping logic is implemented by text shaping engines like HarfBuzz, which is used by virtually every modern application that renders Arabic text (browsers, LibreOffice, Android, iOS).

Transparent Characters

Characters with Joining_Type=Transparent are critical for correct shaping. A Transparent character between two Dual_Joining letters does not break their connection — the joining algorithm "looks through" it. Arabic diacritical marks (tashkeel, like fatha ◌َ U+064E and shadda ◌ّ U+0651) are Transparent, allowing vocalized text to join exactly as the unvocalized base text would.

Zero Width Joiner and Non-Joiner

Two special-purpose characters control joining explicitly:

ZWJ (U+200D, Zero Width Joiner) — Join_Causing: forces a joining connection where it would not otherwise occur
ZWNJ (U+200C, Zero Width Non-Joiner) — causes a break in an otherwise-connecting sequence

Quick Facts

Property	Value
Unicode property name	Joining_Type (jt)
Data file	`ArabicShaping.txt` in UCD
Primary affected scripts	Arabic, Syriac, N'Ko, Mongolian, Hanifi Rohingya
Dual_Joining letters	Most common Arabic consonants (ب، ت، ث، ج…)
Right_Joining letters	Alef (ا), Waw (و), Reh (ر), Zain (ز), Dal (د)
Shaping engine	HarfBuzz (universal), Uniscribe (Windows)
Force join	U+200D Zero Width Joiner
Break join	U+200C Zero Width Non-Joiner

Joining Type