Turns out the venerable plug goes back to at least 1878. Keep in mind, I’m talking about the good old fashioned 1/4″ phone jack with two wires. Over time, the jack and plug have spawned ...

But the basic design of the modular plug was set. New phone designs like the Trimline ... and the modular connectors became known as registered jacks, or RJ for short. Six-conductor RJ11 became ...

While 2.5mm was the original standard for phone headsets ... depending on the number of separate connector rings on the plug and jack. 2.5mm refers to the approximate diameter of the connector.