Version 1.0, 27 May 2016
What is bidirectional text?
Many of the world’s scripts run horizontally from left to right (in lines from top to bottom). Some, like Arabic, Hebrew, and Syriac, predominantly run from right to left. However, these scripts also exhibit left-to-right character direction in the representation of numbers, so they are inherently bidirectional.
We also speak of ‘bidirectional text’ when a text run contains both left-to-right and right-to-left text phrases.
Note: ‘bidi’ is often used as shorthand for ‘bidirectional text’.
Handling of bidirectional text in software is not trivial
While a human can, after very little training, correctly decode most runs of bidirectional text, computer software has traditionally struggled with bidi, or even not supported such text at all. At the time of this writing (May of 2016), software support for bidi has still not settled across the board: some software implementations are very good and stable, such as Adobe InDesign CC and – up to a point – MS Word 2016. But rendering bidi in current web browsers is not always the same.
Enter the Unicode Bidirectional Algorithm
From an early date in the development of the Unicode Standard, around 1999-2000, measures were taken to provide software implementers with standardized tools to support the correct ordering in computer memory, as well as the correct representation on screen and in print of bidirectional text. If you’re not really familiar with the Unicode Bidirectional Algorithm, then before reading further you should read the basic introduction to how the bidi algorithm works.
Required reading
Unicode in XML and other Markup Languages: 3.3 Bidi Embedding Controls
Unicode controls vs. markup for bidi support
Structural markup and right-to-left text in HTML
Inline markup and bidirectional text in HTML
