These have a go gauge on one end and a no-go gauge on the other end. They allow you to inspect internal threads on parts. A taperlock handle lets you quickly replace worn gauges.
Check the minimum internal thread size with the go gauge end, and check the maximum internal thread size with the no-go gauge end. A part is within tolerance if it’s easy to thread the go gauge into the part but difficult to thread the no-go gauge more than two turns.