A Dictionary of Music and Musicians/U. C.
U. C. (Ital. una corda; Fr. petite pedale; Germ, mit Verschiebung). An indication of the use of the left pedal of the pianoforte, by means of which the action is shifted a little to the right, and the hammers made to strike a single string (in modern instruments generally two strings), instead of the three which are ordinarily struck. The return to the use of three strings is indicated by the letters t. c., tre corde, tutte le corde, or sometimes tutto il cembalo. The shifting pedal, the invention of which dates from about the end of the 18th century, is an improvement on the earlier Celeste pedal (also called Sourdine) in which the sound was deadened by the interposition of a strip of leather, or other material, between the hammers and the strings. This arrangement, which is now used only in upright pianos, where from lack of space or from the oblique direction of the strings the shifting action would not be available, gives a dull, muffled sound, which in small instruments is often so weak as to be practically useless; the shifting pedal, on the contrary, produces a beautiful and delicate quality of tone, arising from the sympathetic vibrations of the unused strings, which is by no means the same thing as the ordinary pianissimo, but is of the greatest service in producing certain special effects. Beethoven uses it frequently, in the later Sonatas (from op. 101), and in the Andante of the G major Concerto, op. 58, the whole of which movement is to be played a una corda, except the long shake in the middle, in which Beethoven requires the gradual addition of the other strings, and afterwards the gradual return from three strings to one. His directions are 'due, e poi tre corde,' and afterwards 'due, poi una corda,' but it is not possible to carry them out strictly on the modern pianoforte, as the shifting action now only reduces to two strings instead of one.
In music for string instruments, the direction a una corda is occasionally given, to denote that the passage is to be played upon a single string, instead of passing from one string to the next, in order to avoid any break in the quality of tone produced. [See also Pedals, Sordini, Verschiebung.]