Thacher's Calculating Instrument, patented in 1881, can be thought of as a series of conventional slide rules mounted around the periphery of a cylinder. There are 20 pairs of scales forming the outer framework of the cylinder. Within this framework there is a solid cylinder that can be both rotated and moved longitudinally within the framework of scales. The open framework itself can also be rotated as a unit. Thacher's Calculating Instrument was mounted on a mahogany base and produced by the Keuffel and Esser Company of New York.