There are several different possible wall configurations which include {HEAD ~ combinations} of {TAIL ~ single walls} and double walls 
of
There are several different possible wall configurations which include { HEAD ~ combinations } of { TAIL ~ single walls } and double walls
OTH OTH OTH OTH OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD REL TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH