Bi-directional text is text containing text in both text directionalities, both 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 directionality in each row.
Some writing systems of the world, notably the Arabic (including variants such as Nasta'liq), Persian and Hebrew scripts, are written in a form known as right-to-left (RTL), in which writing begins at the right-hand side of a page and concludes at the left-hand side. This is different from the left-to-right (LTR) direction used by most languages in the world. When LTR text is mixed with RTL in the same paragraph, each type of text is written in its own direction, which is known as bi-directional text. This can get rather complex when multiple levels of quotation are used.
Many computer programs fail to display bi-directional text correctly. For example, the Hebrew name Sarah (שרה) is spelled shin (ש) resh (ר) heh (ה) from right to left. Some Web browsers may display the Hebrew text in this article in the opposite direction.
Languages using bi-directional text
There are very few scripts that can be written in either direction.
Such was the case with Egyptian hieroglyphics, where the signs had a distinct "head" that faced the beginning of a line and "tail" that faced the end.
Chinese characters can also be written in either direction as well as vertically (top to bottom then right to left), especially in signs (such as plaques, but the orientation of the individual characters is never changed). This can often be seen on tour buses in China, where the company name customarily runs from the front of the vehicle to its rear - that is, from right to left on the right side of the bus, and from left to right on the left side of the bus.
The right side (text runs from right to left)
The left side (text runs from left to right)
The right side of a China Southern Airlines aircraft. The text runs from right to left ( 空 航 方 南 国 中 ).
The left side shows the text running from left to right ( 中 国 南 方 航 空 ).
A photo that shows text on both sides of a China Post vehicle (thanks to the open door)
Another variety of writing style, called boustrophedon, was used in some ancient Greek inscriptions, Tuareg, and Hungarian runes. This method of writing alternates direction, and usually reverses the individual characters, on each successive line.
Bidirectional script support is the capability of a computer system to correctly display bi-directional text. The term is often shortened to the jargon term BiDi or bidi.
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