Linkage-type mechanical analog computers use whippletree linkages to add and subtract quantities represented by straight-line motions. The illustration here of whippletrees for a three-animal team is very similar to a group of linkage adders and subtracters: "load" is the equivalent of the output sum/difference of the individual inputs. Inside the computer, cylinders on the knob shafts have thin metal tapes wrapped around them to convert rotary to linear motion.