The block usually includes a plurality of interconnected gas containing tunnels or passages which form a closed loop path in {HEAD ~ the shape} of a triangle, a rectangle, or {TAIL ~ any polygonal path} 
The block usually includes a plurality of interconnected gas containing tunnels or passages which form a closed loop path in { HEAD ~ the shape } of a triangle , a rectangle , or { TAIL ~ any polygonal path }
OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD REL OTH OTH OTH OTH OTH OTH OTH TAIL TAIL TAIL TAIL TAIL TAIL TAIL