What it sounds like, and the records to hear it on.
The Mellotron is an electro-mechanical, polyphonic keyboard originally developed and built in Birmingham, England in the early 1960s. The Mellotron superseded the Chamberlin, which was the world's first sample-playback keyboard. The heart of the instrument is a bank of magnetic audio tapes (which are parallel linear, not looped as has sometimes been reported or presumed). Each tape has approximately eight seconds of playing time. Playback heads (underneath each key) enable the playing of pre-recorded sounds. .