In 1949, Crosby gave Paul one of the first Ampex Model 200A reel to reel tape recorders. Paul invented Sound on Sound recording using this machine by placing an additional playback head, located before the conventional erase/record/playback heads. This allowed Paul to play along with a previously recorded track, both of which were mixed together on to a new track. This was a mono tape recorder with just one track across the entire width of quarter-inch tape; thus, the recording was "destructive" in the sense that the original recording was permanently replaced with the new, mixed recording. He eventually enhanced this by using one tape machine to play back the original recording and a second to record the combined track. This preserved the original recording.