Your headphone jack began life on a nineteenth-century telephone switchboard
The plug on a guitar cable and the one on old headphones share an ancestor: the connectors operators jammed into telephone switchboards in the late 1870s. The very word jack comes from an 1879 patent whose spring-loaded socket reminded people of a folding pocket knife.
The first telephone switchboard went into service in Boston in 1877, and a year later George W. Coy opened the first commercial manual exchange in New Haven. In 1879 Charles E. Scribner patented a spring-jack switch: a lever held against one contact until a plug pushed it aside and broke the connection. Because it resembled a clasp knife it was called a jack-knife, which is said to be where the term jack began. Henry P. Clausen's 1901 patent gave the quarter-inch plug the shape still used on audio gear. Western Electric, the Bell System's manufacturing arm, refined the designs, which later spread to the US military and other industries.
Every plug has a tip, a sleeve nearest the handle and up to three rings in between, all separated by insulating bands. The sleeve usually serves as the shared ground, so a plug always carries one fewer signal than it has contacts. Two contacts give mono, three can carry stereo or a balanced mono signal, and four-contact versions add a microphone or video.
The smaller mini size was created in the 1950s for earpieces on transistor radios, and Sony's EFM-117J radio of 1964 helped popularise it. It truly took off with the Walkman in 1979, which had no speaker of its own, so listening meant plugging in headphones. An even smaller sub-mini size turned up on portable tape recorders, where it carried the on-off switch from the microphone, and later on mobile phone headsets.
The naming is a tangle. Audio engineers say quarter-inch to tell it apart from XLR, the British say jack plug, and some confuse it with the unrelated RCA phono plug or with RJ11 telephone wall sockets. Standards bodies have not settled things either: one mechanical engineering society simply calls whichever part is more fixed the jack, regardless of which piece is male.
Source: Phone connector (audio)
