As {HEAD ~ carriage} 3 moves along {TAIL ~ slot} 7, pointer 16 will move relative to a scale 4 imprinted on or otherwise carried by frame 2 
As { HEAD ~ carriage } 3 moves along { TAIL ~ slot } 7 , pointer 16 will move relative to a scale 4 imprinted on or otherwise carried by frame 2
OTH HEAD HEAD HEAD HEAD HEAD OTH REL REL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH