C trigraph

In the C family of programming languages, a trigraph is a sequence of three characters, the first two of which are both question marks, that represents a single character.The reason for their existence is that the basic character set of C is a subset of the ASCII character set (nine of its characters lie outside the smaller ISO 646 invariant character set). This can pose a problem for writing source code if the keyboard being used does not support any of these nine characters. The ANSI C committee invented trigraphs as a way of entering source code using keyboards that supported any version of the ISO 646 character set. Non-ASCII ISO 646 character sets are not much used today, but trigraphs remain in the C99 standard.
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