3.5 headphone jack for over 100 years, and it is still used

this connector exists almost as much as telephones; its initial use was in 19th century switchboards, when human operators manually connected your call.

They needed a way to easily make and break the electrical connection that the audio carries. Today's use of the jack is almost the same, even if most of the sound lives in a digital domain (of course, it will inevitably convert to an analog signal before it reaches your ears).

The telephone jacks were several sizes. The 19th century was 1/4 inch (6.35 mm), but 3.5 mm jacks are more common today. Called “miniature sizes,” they were first used to connect headphones to the 1950s new-fangled transistor radios.

A second boost in popularity came with the original Sony Walkman. He revolutionized the world of portable audio, and he (along with many subsequent cassette and later CD players) made sure everyone had at least one pair of headphones with a 3.5 mm jack.