The process of combining MIDI messages transmitted from two or more MIDI devices into one coherent MIDI data stream so that the messages appear to have been generated by only one device. This is not just connecting MIDI cables as MIDI messages are structured and this structure needs to be preserved. For example, if two Note On messages arrive simultaneously at the two inputs, the merge device will have to store one of them in a buffer until the first is sent. Because MIDI messages are variable in length, and because real-time massages have to take a priority, the merge device must be able to identify and distinguish between different data types. This generally requires a separate microprocessor, making a MIDI merge unit more expensive than a MIDI Thru device.