A parameter found in many keyboards, synthesizers, and drum machines that have built in sequencers. It allows the user to synchronize the sequencer in the device to some external sequencer or clock source. The other choice is obviously internal sync, where the device uses its own clock and tempo information for timing. When set to external sync (sometimes called MIDI sync) it essentially becomes a slave and waits for MIDI clock signals to come in over MIDI, which it then follows. For example, one could program a drum part in a drum machine, but do all of the keyboard parts in the sequencer of a keyboard while the sequencer is slaved to the drum machine’s MIDI clock. When the user starts the drum machine, MIDI clock signals are transmitted to the sequencer in the keyboard, which makes it follow in time. Tempo settings need not be set in the slave device because it will simply follow the master. This is all distinctly different that synchronizing over MIDI using MIDI Time Code (MTC) where the only thing transmitted and received is a raw timing signal that has no relationship with the music. In that case tempo and song start time must be set in the slave device.