When {HEAD ~ the fifth wheel coupler} is connected to the pulley system, the coupler can be moved into and out of engagement with {TAIL ~ the king pin} 
When { HEAD ~ the fifth wheel coupler } is connected to the pulley system , the coupler can be moved into and out of engagement with { TAIL ~ the king pin }
OTH HEAD HEAD HEAD HEAD HEAD HEAD HEAD HEAD OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH TAIL TAIL TAIL TAIL TAIL TAIL TAIL