The block comprises {HEAD ~ a front surface} and 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 
The block comprises { HEAD ~ a front surface } and 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 HEAD HEAD HEAD HEAD HEAD HEAD HEAD OTH OTH OTH OTH 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