Many computer programs fail to display bi-directional text correctly. For example, the Hebrew Tetragrammaton (יהוה) should be spelled yodh(י) heh(ה) waw(ו) heh(ה) from right to left. Some web browsers may display the Hebrew text in this article in the opposite direction.
Very few languages may 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.
Early computer installations were designed only to support a single writing system, typically for left-to-right scripts based on the Latin alphabet only. Adding new character sets and character encodings enabled a number of other left-to-right scripts to be supported, but did not easily support right-to-left scripts such as Arabic or Hebrew, and mixing the two was not practical. It is possible to simply flip the left-to-right display order to a right-to-left display order, but doing this sacrifices the ability to correctly display left-to-right scripts. With bidirectional script support, it is possible to mix scripts from different languages on the same page, regardless of writing direction.
In particular, Unicode provides complete BiDi support, with detailed rules as to how mixtures of left-to-right and right-to-left languages are to be encoded and displayed. In Unicode encoding, all characters are stored in writing order, and software works out which direction on the page or screen the script should be displayed.