Woodpeckers SSPRO-CA Story Stick Caliper Arms
Caliper arms allow you to quickly create a physical stick to use as a cut-to reference. The arms are offset to the inside from one direction and the outside from the other. The inside offset is used for the inside measurements and the outside offset for outside measurements.
Say you want to put a perfectly fitting shelf between two cabinet sides. Simply adjust the arms out until they contact the sides, lock them in place and use that physical example to set your saw fence that distance from the blade.