Legend
  1 - Main barrel
  4 - Adjusting rod (a tab on the rod, not shown, fits into a slot on the barrel to prevent them from rotating relative to each other; they are confined to sliding in and out)
10 - Female screw cut into tube 1 (orange); the example uses a right hand, 20 threads per inch (tpi) pitch
11 - Male screw cut onto rod 4 (blue); the example uses a right hand, 25 tpi thread
12 - Sleeve cap or thimble
12x -Milled bead on thimble
13 - Nut sleeve with, for example, outer 20 tpi threads (orange) and inner 25 tpi threads (blue); fixed attachment to sleeve cap 12
14 - Beveled end of sleeve cap with ruler markings to show rotational movement

Each full rotation of the thimble + nut sleeve, 12 + 13, moves the rod 0.01 in (0.25 mm) relative to the main barrel due to the differential motion from the 20 tpi and 25 tpi threads.

1. ↑ US patent 585184 (1897-06-29) Clarke, George R. Calipers