A ring gauge is used to check external diameters on cylindrical objects, as well as calibrating other gauges or standards such as calipers, inside micrometers or bore gauges. Generally, they are cylindrical rings made of a stable material such as steel, with a precision finished interior surface and a known tolerance.
Plug gauges, in some cases called pin gauges, are metrology tools whose purpose is to gage the inside diameters of holes that have been drilled or machined into a manufactured part, component, or assembly.