The Moore & Wright Firm Joint Inside Calipers are manufactured from polished tool steel and have a large headed nut and screw assembly to provide a firm and rigid friction joint, with no play. The sizes listed are the effective leg length which is measured from the joint assembly. The Firm Joint Inside Calipers are manufactured to BS3123 standards. Note: Firm joint caliper capacities are generally 1/3 greater than the effective leg length. Ref: UTTK3123