The block comprises a front surface and {HEAD ~ a back surface} adjoined by {TAIL ~ first and second side surfaces}, a top surface and a bottom surface each lying adjacent the front, back, and first and second side surfaces 
adjoined by
The block comprises a front surface and { HEAD ~ a back surface } adjoined by { TAIL ~ first and second side surfaces } , a top surface and a bottom surface each lying adjacent the front , back , and first and second side surfaces
OTH OTH OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD HEAD REL REL TAIL TAIL TAIL TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH