Standard MIDI File (SMF)
The file format behind .mid — timestamped MIDI messages, so a sequence can move between programs and machines.
The 1983 MIDI standard only described the cable; how to save a song was left to every sequencer vendor. The MIDI Manufacturers Association published SMF in 1988, based on a format from Dave Oppenheim at Opcode. A file is a series of chunks: MThd states the format and the timing resolution in ticks per quarter note, and each MTrk holds events whose delta times are encoded as variable-length integers.
Format 0 puts everything on one track, format 1 holds several parallel tracks, and format 2 — independent patterns — is used by almost nobody. Meta events carry tempo, time signature, text and lyrics, which is how .kar files became karaoke. The format has survived nearly unchanged for close to forty years and is imported by every DAW, usually alongside the General MIDI instrument map.