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