Abstract: A pair of telescoping elongated prisms locked against separation provide a series of size indications by dash numbers for standardized O-rings in a series of windows. Each face of the inner prism has a semicylindrical projection which matches a similar projection on the outer prism and rests up against it to form what looks like a stationary pulley wheel. Also, the inner prism is provided with diameter-determining grooves. The outer prism also has on each face, or most faces, a projection that looks like a pulley wheel and is the same size as that formed by the matching semi-cylindrical projections and is spaced away from it.