Furthermore, {HEAD ~ the separation} between a pair of adjacent rows need not be equal to the separation between another pair of {TAIL ~ adjacent rows} 
Furthermore , { HEAD ~ the separation } between a pair of adjacent rows need not be equal to the separation between another pair of { TAIL ~ adjacent rows }
OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH TAIL TAIL TAIL TAIL TAIL TAIL