The even field and {HEAD ~ the odd field} combine to form {TAIL ~ one frame} of TV timing 
combine to form
The even field and { HEAD ~ the odd field } combine to form { TAIL ~ one frame } of TV timing
OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD HEAD REL REL REL TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH