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