An output on a synth or device that exactly duplicates whatever data is being received at the MIDI input. The MIDI thru allows a second device to effectively be connected to the same MIDI cable, by running a cable from the MIDI thru of the first device to the MIDI input of the second device; both devices will see the same data. If the second device also has a MIDI thru, then a third device can be connected, and so on; however, since each MIDI thru adds latency, it is usually not recommended to connect more than three devices in a chain. Unless it is the intent that all devices in the chain respond to all data that is sent, then the channel assignments of the devices must be laid out so that each device will respond only to the data intended for it. Compare with MIDI overflow.