Editing Cassette deck (section)

==== Three heads for realtime monitoring of recordings and improved sound quality ====
Three-head technology uses separate heads for recording and playback (the third of the three heads being the erase head). 
This allows different record and playback head gaps to be used. 
A narrower head gap is optimal for playback than for recording, so the head gap width of any combined record/playback head must necessarily be a compromise.  
Separate record and playback heads also allow off-the-tape monitoring during recording, permitting immediate verification of the recording quality. 
(Such machines can be identified by the presence of a ''monitor'' switch with positions for ''tape'' and ''source'', or similar.) 
Three-head systems were common on reel-to-reel decks, but were more difficult to implement for cassettes, 
which do not provide separate openings for record and play heads. 
Some models squeezed a monitor head into the capstan area, and others combined separate record and playback gaps into a single headshell.