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What Is the Bidirectional Category?

The Bidi Class (formally Bidi_Class, also called bidirectional category) is a Unicode property that controls how characters are positioned in a line of mixed left-to-right (LTR) and right-to-left (RTL) text. It is the primary input to the Unicode Bidirectional Algorithm (UBA, described in Unicode Standard Annex #9), which determines the visual display order of characters in a paragraph.

Every code point is assigned one of 23 Bidi Class values. The algorithm uses these values—along with explicit directional override characters—to resolve the correct rendering order for Arabic mixed with English, Hebrew mixed with numbers, or any other bidirectional combination.

The Major Bidi Class Values

import unicodedata

chars = [("A", "Latin"), ("ب", "Arabic"), ("5", "Digit"),
         ("\u200F", "RLM"), ("\u200E", "LRM")]

for char, label in chars:
    bc = unicodedata.bidirectional(char)
    print(f"  {label:12} U+{ord(char):04X}  Bidi={bc}")

# Latin        U+0041  Bidi=L
# Arabic       U+0628  Bidi=AL
# Digit        U+0035  Bidi=EN
# RLM          U+200F  Bidi=R
# LRM          U+200E  Bidi=L

Why It Matters in Practice

Without correct bidi handling, a string like "Hello مرحبا World" will display with the Arabic word in the wrong position or with punctuation displaced. HTML provides dir attributes and the Unicode characters U+200F (RLM), U+200E (LRM), and the bidi isolate characters (U+2066–U+2069) to guide the algorithm. Web developers working with RTL content must understand that the visual order of characters on screen differs from their logical (storage) order.

Quick Facts