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