The Jack (connector) reference article from the English Wikipedia on 24-Apr-2004
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Jack (connector)

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In electronics, a jack is a socket. In some cases it is arbitrary which of a pair of mating connectors is the plug, and which the socket (see plug). The term jack occurs in several related terms:

When used with a size, the term jack refers to the socket that matches the corresponding size of jack plug. For example:

A headphone jack or earphone jack is normally one of the three standard sizes of jack, but the term could refer to any socket used for this purpose.