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A bidirectional text contains two text directionalities, right-to-left (RTL) and left-to-right (LTR). It generally involves text containing different types of alphabets, but may also refer to boustrophedon, which is changing text direction in each row.
An example is the RTL Hebrew name Sarah: שרה, spelled sin (ש) on the right, resh (ר) in the middle, and heh (ה) on the left. Many computer program failed to display this correctly, because they were designed to display text in one direction only.
Some so-called right-to-left scripts such as the Persian script and Arabic are mostly, but not exclusively, right-to-left—mathematical expressions, numeric dates and numbers bearing units are embedded from left to right. That also happens if text from a left-to-right language such as English is embedded in them; or vice versa, if Arabic is embedded in a left-to-right script such as English.