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