{HEAD ~ A driving linkage} is coupled between the barriers and {TAIL ~ the parallelogram linkage}, such as a simple bell crank linkage 
is coupled between
{ HEAD ~ A driving linkage } is coupled between the barriers and { TAIL ~ the parallelogram linkage } , such as a simple bell crank linkage
HEAD HEAD HEAD HEAD HEAD HEAD HEAD REL REL REL OTH OTH OTH TAIL TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH OTH OTH OTH