This type of spring caliper is used to measure thickness, distance, or diameter of a particular item. It is used in many fields of work and study: construction, plumbing paleontology, forensic medicine, just to name a few. Just loosen the screw, hold the object to be measured, and tighten the screw. Then measure the opening size against a flat ruler to determine the size of the object. The larger the object, the bigger size caliper you should use.* This is a 18 inch size outside spring-type caliper - should choose the appropriate size as needed* Open And Close The Caliper With The Tension Screw On The Side - For Comparison Measurements* The Tension Screw Is Used To Hold The Caliper In The Desired Position For Measuring* You Can Measure Distance, Diameter Or Thickness Of A Particular Item* Made Of Durable Steel And Useful In Many Work And Study Applications