The assembly of heads in a tape recorder. This generally consists of two or three tape heads carefully positioned on a head block, which is usually partly placed within a metal housing to help shield against EMI. In two head configurations there is an erase head (for erasing the tape just before recording) and a record/play or sync head. In three head configurations there is an erase head, a record head, which may also be able to play and act as a sync head, and a third head known as a repro (short for reproduce) head, which is dedicated for playback. In some designs the repro head is optimized for playback and has better playback performance than a record or sync head could. The sync head gets its name because of the convenience it provides for overdubbing. It enables the engineer and/or musicians to be able to hear (play back) existing tracks in time (sync) with new tracks being laid down (recorded).