Short for MIDI Through. The MIDI through is a connection available on many MIDI devices. It’s purpose is to pass on (or through) an exact copy of the data present at the MIDI In of the device. This is distinct from a MIDI out, which can sometimes pass on a copy of the input, but usually has other information on it generated by the device in question. MIDI Thru allows many MIDI devices to have their MIDI connections daisy chained together all being driven by a common source or controller, which makes building complex systems much easier.