The Moore & Wright Firm Joint Outside Caliper are manufactured from polished tool steel and have a large headed nut and screw assembly that provides 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 Outside Calipers are manufactured to BS3123 standards. Note: Firm joint caliper capacities are generally 1/3 greater than the effective leg length.The Moore & Wright 33118 Firm Joint Outside Caliper has the following specification: Size: 450mm (18in).