A ring gauge (AKA thread gauge or thread ring gauge) is a type of precision measuring tool used to perform accuracy tests on the dimensions of externally threaded items such as bolts, nuts and screws. Ring gauges have a center hole that is internally threaded and accepts the part being tested. If the part fits into the hole, it is the correct diameter. Ring gauges also determine the accuracy of pitch diameter. Zoro supplies hundreds of ring gauges. Items vary by brand, thread size, and type of certificate of accuracy, among other considerations. Another precision measuring tool used to inspect and perform accuracy tests on threads is called a plug gauge. Plug gauges are used to test internal threads.