Program Change (MIDI)
The MIDI message that tells a synth to switch sounds — two bytes long, and the source of one of MIDI's most durable off-by-one errors.
The status byte is Cn, n being the channel, followed by a single data byte: program 0–127. That is all that fits, and 128 sounds did not stay enough for long. So the convention is to send Bank Select first, controllers 0 and 32 in MIDI CC, followed by the program change — just over two million addressable patches on paper.
The off-by-one is in the numbering. The protocol counts 0–127, but nearly every display and manual shows 1–128, so "program 25" in a manual may mean byte 24 or 25 depending on who wrote it. General MIDI settled what the numbers mean, but not where they start. On stage the message switches pedals and patches between songs with one footswitch.