Una corda

Una corda (It.: one string) (Ger.: Verschiebung), a piano stop, a device for shifting the keyboard so that only one string of each pair of unisons was struck by the hammer. Bartolommeo Cristofori's piano of 1726—his later model—is already provided with such a contrivance. In England and the U.S. this stop was subsequently called “piano,” and for a while it was also known in England as “harmonica” (92). Harding, Rosamund E. M. The Pianoforte. Cambridge, 1933.