Spirit level extenders 9 have {HEAD ~ a pair} of {TAIL ~ opposite inwardly directed longitudinal female dovetail flanges} 17 for engaging a spirit level clamp 11  
of
Spirit level extenders 9 have { HEAD ~ a pair } of { TAIL ~ opposite inwardly directed longitudinal female dovetail flanges } 17 for engaging a spirit level clamp 11
OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD REL TAIL TAIL TAIL TAIL TAIL TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH OTH OTH OTH OTH