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