Les Paul, the father of multi-track recording, also invented an early overdubbing technique, which he called sound-on-sound. He asked Ampex, the company that introduced the first American tape recorder in 1947, to modify one of their mono tape Paul had them put a second playback head, just ahead of the erase head. Here’s how it worked. Les Paul would record the first guitar part and then rewind the tape. He’d then hit the record button. The tape would playback from the upstream play head, and he would mix that signal with a new part that he was playing live. Then both parts would be recorded at the record head. He would repeat this process as many times as necessary to get all of the parts ‘s the downside to this technique, and it’s a big one. If he made a mistake, or the mix wasn’t right, he’d have to start all over again on the first pass, since every time the tape was played back, everything recorded previously was also being erased. Les Paul called it “burning bridges”, but even with the pressure to be perfect on every take, Les Paul and his wife Mary Ford recorded many hit records using his sound-on-sound technique. No wonder he wanted multi-track.