6.35mm TRS

The 6.35mm connector's official name is simply the "phone connector" -- a direct reference to its origin in manual telephone switchboards, not a later marketing name. Early switchboard plugs trace to the first commercial switchboard installed in Boston in 1877 and a New Haven switchboard credited to George W. Coy in 1878; Charles E. Scribner patented related switchboard plug designs between 1878 and 1882, and Henry P. Clausen's 1901 patent established the plug construction still in use today. Western Electric, the Bell System's manufacturing arm, refined and mass-produced the design that audio equipment makers later adopted wholesale.

At this size, TRS splits between two roles: balanced mono line or microphone connections in professional audio gear, or stereo signals in full-size headphones -- the same tip-ring-sleeve logic as the smaller 3.5mm TRS connector, just at four times the shaft diameter.