MIDI Thru
The third DIN socket on classic MIDI gear, sending out an exact copy of whatever arrives at MIDI In.
MIDI 1.0 from 1983 has no network addresses, just sixteen channels on a one-way cable. The Thru port lets you chain instruments: a keyboard drives a synth, which passes the data on to a drum machine listening on its own channel. Thru is not the same as Out — Out sends what the device itself plays, Thru only what it heard.
Each hop goes through an optocoupler that smears the signal edges slightly, so long chains can start dropping or garbling messages. The fix was thru boxes and patchbays that split one input to many outputs in parallel. The software equivalent, "soft thru", forwards whatever arrives on one MIDI interface to another port.