Multitrack recording
Recording instruments onto separate, synchronised tracks so they can be replaced, added to and mixed later.
Before multitrack, a song was captured in one piece: the whole band in front of the same microphones, and one wrong note meant another take. The crucial trick came from Ampex in 1955. Sel-Sync let the record head double as a playback head on the tracks already recorded, so a musician could hear earlier takes in time and lay a new part on top — an overdub. Les Paul received one of the first eight-track machines in 1957 and played it like an instrument.
Things moved quickly after that. Four tracks were enough for Sgt. Pepper in 1967, but only by merging tracks and freeing them up along the way. Sixteen and then twenty-four tracks became studio standard in the 1970s, ADAT made eight digital tracks affordable in 1992, and a DAW now offers more tracks than anyone can be bothered to use.