Calipers

A pincher-like measuring device in which two pins or arms are snugged up against the edges of the part to be measured. In pottery, a useful caliper consists of two slightly curved and hook-ended arms pinned together in the center, resembling an X. One end is shaped for measuring the inside of things, the other the outside, AND . . . when you make a measurement with one end, the other end is set to the mirroring inside or outside measurement. A typical use is to measure the rim (across the inside) of a pot where a lid will sit with one end, and then use the measurement given by the other end to make sure the distance (across the outside) of the lid matches when it is made.

Since clay shrinks as it dries, make sure the parts you want to fit together are the same wetness when you measure them. If you want a lid to fit a pot, measure the pot's rim as soon as you've thrown it, not when it's leather hard and trimmed.