Plug gauges are used to determine if tenons and receivers are true. Check by inserting the correct size plug gauge into the receiver. Check concentricity by looking for consistent feel when inserting the gauge. Check roundness by noting gaps between the part and the gauge. Refit and re-round by tapping on the gauge or the exterior of the part. Most helpful with metal joints such as those on flute tenons and saxophone necks, but has many uses throughout the shop.