The underside of each channel base provides {HEAD ~ a relatively large lateral dimension} for fastening {TAIL ~ individual channels} together by means of a transverse stringer or strut 
for fastening
The underside of each channel base provides { HEAD ~ a relatively large lateral dimension } for fastening { TAIL ~ individual channels } together by means of a transverse stringer or strut
OTH OTH OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD HEAD HEAD HEAD REL REL TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH OTH OTH OTH OTH