There are two types of MIDI Thru. One, a simple hardware connection, is found on the back panels of many synthesizers. The Thru jack, in this case, simply duplicates whatever data is arriving at the MIDI -In jack, being a hard-wired connection between the two. Compare with MIDI Echo. Sequencers have a second type, called a Software Thru. In this case, data arriving at the In-jack is merged with data being played by the sequencer, and both sets of data appear in a single stream at the Out- (not the Thru-) jack. A Software Thru is useful because it allows a master keyboard to be hooked up to the sequencer’s MIDI input and a tone module to its output. The keyboard can they play and produce sound through the tone module, and the sequencer can also send its messages directly to the tone module. Also called a MIDI Splitter. See also MIDI Out/Thru.