The first and second channels each have a flange extending from one wall to {HEAD ~ adjacent opposed wall} to define {TAIL ~ the elongated opening} into the channel 
to define
The first and second channels each have a flange extending from one wall to { HEAD ~ adjacent opposed wall } to define { TAIL ~ the elongated opening } into the channel
OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD HEAD REL REL TAIL TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH