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