{HEAD ~ means} for supplying {TAIL ~ two kinds} of clock signals having the same frequency and a phase difference of 180 degrees; and 
for supplying
{ HEAD ~ means } for supplying { TAIL ~ two kinds } of clock signals having the same frequency and a phase difference of 180 degrees ; and
HEAD HEAD HEAD HEAD HEAD REL REL TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH