In {TAIL ~ use}, the blocks may be stacked to provide {HEAD ~ an interlocking structure} wherein the protrusions of one block interfit within the insets of another block 
In { TAIL ~ use } , the blocks may be stacked to provide { HEAD ~ an interlocking structure } wherein the protrusions of one block interfit within the insets of another block
OTH TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD HEAD OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH