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