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linguistics, a prefix is a type of affix that precedes the morphemes to which it can attach. In English, most prefixes are bound morphemes, meaning that they cannot occur as independent words (excluding citational uses, e.g., saying "Speaking of the prefix, 'un-',...").
Associative prefixes shows an association. For examples in Old English and German, ge- has the parallel semantics as the Latin com-, such as indicating:
In the syntax of notations used in mathematics and computer science, prefix is used to describe an operator such as the usual addition sign + that is taken to bind to the variables succeeding them. See operator for more on the placement of operators.
A telephone prefix or area code is the first three digits of a seven-digit telephone number. It shows which exchange the remaining numbers refer to. For example: abc-defg (with actual numbers) might refer to Anytown while cab-defg could refer to Anycity.
Some places restrict certain prefixes to only fax numbers or for cell phones while in other places the prefixes are all jumbled up.
555 (at least in North America) is an impossible prefix. For this reason, it is often used for phone numbers in television and movies. Failure to do so in the film Bruce Almighty resulted in someone's real phone number being used, eventually leading to a lawsuit.