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 Bidirectional Category?

Property determining how a character behaves in bidirectional text (LTR, RTL, weak, neutral). Used by the Unicode Bidirectional Algorithm to determine display order.

RTL (Right-to-Left) — Unicode Glossary

Q: 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.

What is RTL (Right-to-Left)?

RTL stands for Right-to-Left, describing text that is written and read from right to left. Arabic, Hebrew, Persian (Farsi), Urdu, and several other scripts are inherently RTL. This is not a stylistic choice — it is the fundamental direction of these writing systems, as fixed by their orthographic traditions dating back thousands of years. Proper RTL support requires changes at every layer of text technology: from Unicode's bidirectional algorithm to CSS properties to HTML markup.

Unicode Bidirectional Algorithm (Bidi)

The Unicode Bidirectional Algorithm (UAX #9) is the specification for how to display text that mixes LTR and RTL content — called bidirectional or bidi text. Because a single paragraph might contain English, Arabic, and numbers all together, a strict mechanical rule set is needed to determine the display order of each run.

The algorithm assigns every character a bidi category: - L (Left-to-Right): A–Z, Latin digits, most punctuation - R (Right-to-Left): Hebrew characters - AL (Arabic Letter): Arabic, Thaana, Syriac characters - AN (Arabic Number): Arabic-Indic digits - EN (European Number): 0–9 ASCII digits

The algorithm then resolves the visual ordering based on the paragraph's base direction and these per-character categories.

Bidi Control Characters

Unicode provides invisible characters to explicitly override the automatic bidi behavior:

Character	Unicode	Name	Effect
RLM	U+200F	Right-to-Left Mark	Forces bidi context rightward
LRM	U+200E	Left-to-Right Mark	Forces bidi context leftward
RLE	U+202B	Right-to-Left Embedding	Start RTL embedding
LRE	U+202A	Left-to-Right Embedding	Start LTR embedding
RLO	U+202E	Right-to-Left Override	Force all text RTL
LRO	U+202D	Left-to-Right Override	Force all text LTR
PDF	U+202C	Pop Directional Formatting	End an embedding/override
RLI	U+2067	Right-to-Left Isolate	RTL bidi isolate (modern)
LRI	U+2066	Left-to-Right Isolate	LTR bidi isolate (modern)
PDI	U+2069	Pop Directional Isolate	End isolate

The RLO character (U+202E) was notoriously abused in filenames to disguise file extensions: Vacation‮gpj.exe would display as Vacationgpj.exe visually.

RTL in HTML and CSS

<!-- Set paragraph direction -->
<p dir="rtl">مرحبا بالعالم</p>

<!-- HTML attribute on document root -->
<html lang="ar" dir="rtl">

<!-- Use the <bdi> element for user-generated content of unknown direction -->
<p>Posted by <bdi>محمد</bdi> yesterday</p>

/* CSS direction property */
.arabic-text {
  direction: rtl;
  text-align: right; /* or use text-align: start (logical) */
}

/* CSS logical properties (preferred for bidi) */
.box {
  margin-inline-start: 1em;  /* left in LTR, right in RTL */
  padding-inline-end: 0.5em; /* right in LTR, left in RTL */
  border-inline-start: 2px solid; /* left border in LTR */
}

Mirroring in RTL

When a UI is mirrored for RTL, directional icons must also be mirrored: a "forward" arrow in LTR points right (→), but in RTL it should point left (←). Layout elements like sidebars, toolbars, and scrollbars typically mirror position.

Quick Facts

Property	Value
RTL scripts	Arabic, Hebrew, Persian, Urdu, Thaana, Syriac, N'Ko, Tifinagh
Unicode bidi spec	UAX #9 (Unicode Standard Annex 9)
HTML direction attribute	`dir="rtl"` or `dir="ltr"`
CSS property	`direction: rtl`
Right-to-Left Mark	U+200F (RLM)
Right-to-Left Override	U+202E (RLO) — security risk
Modern isolate characters	RLI (U+2067), LRI (U+2066), PDI (U+2069)
CSS logical properties	`margin-inline-start`, `padding-inline-end` for bidi-aware layout

RTL (Right-to-Left)