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