Abstract: A transparent face plate has inscribed thereon a first circle surrounded by graduations which divide it into zero through 360.degree.. Two lines of transparency, which are denoted sine and cosine, respectively, intersect at right angles at the center of the circle, and at opposite sides of the center are divided into decimal graduations 0.01 through 1.0. Mounted to rotate beneath the base plate is a disc having inscribed on its face an indicator circle having a radius equal to one half of that of the first-named circle, and located in offset relation to the latter, so that the indicator circle extends at one diametral side through the center of the first circle, and at its opposite diametral side registers tangentially with said first circle.