The frame may be rectangular, when {HEAD ~ each side} comprises at least one projection, {TAIL ~ typically a lug}, or at least one longitudinal array of loops 
comprises
The frame may be rectangular , when { HEAD ~ each side } comprises at least one projection , { TAIL ~ typically a lug } , or at least one longitudinal array of loops
OTH OTH OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD REL OTH OTH OTH OTH OTH TAIL TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH OTH OTH OTH OTH