MIDI Beat Clock, often called MIDI Clock, is the standard for tempo based sequencing. When you want to sync the tempo and song position of two or more MIDI devices this is what you want to use. One Device is the "Master" and all other machines are slaves. The "Master" generates the time base and the slaves listen for the time base and when they hear it they respond appropriately. For clarity it should be stated that MIDI Beat Clock is not MIDI Time Code (MTC) nor is it MIDI Machine Control (MMC). More on these later. Some devices support "Song Position Pointer". With Song Position Pointer enabled, MIDI Beat Clock will nicely sync just about all your MIDI devices. Some devices do not respond to song position pointer. All this means is that you will have to rewind each device and press play on the Master device to start the song.

MIDI Time Code is the MIDI standard for Time Based syncronization. This is what you should use if you are scoring movies, if you need to sync to a digital multi-track recorder or most situations where time is what is important.

MIDI Machine Control is mostly used to control the transport of another machine. The transport is the collection of buttons including "Play, Stop, Record, Rewind, Fast Forward, and Pause." It's important to understand that MMC is seperate from MTC and MIDI Clock. MMC does not sync devices. One could control a complex MIDI rig including a computer sequencer set as the master from a keyboard that is sending MMC messages to the computer. In fact this is pretty common.