Calipers in bow spring joint or vernier styles accurately measure interior or exterior diameters. The adjustable nut in the bow spring joint caliper offsets the spring, allowing the leg position to be determined. While these calipers need to be used in conjunction with a separate ruler, vernier styles have an all in one design. After moving the adjustable jaw from the fixed one, tighten the clamp screws to lock in the precise distance reading.