una corda
The left pedal on a piano, shifting the hammers to strike fewer strings for a softer tone.
In Depth
Una corda means one string and refers to the left pedal on a grand piano. When depressed, the entire keyboard action shifts slightly to the right, so the hammers strike two strings instead of three (or one instead of two in the lower register). This changes both the volume and the tonal colour.
The effect is not simply quieter — it produces a distinctly different timbre: warmer, more veiled, and more intimate. On Beethoven's and Schubert's pianos, the shift was greater, and the hammers could actually strike only one string per note (hence the name). Modern grand pianos typically shift to two strings, so the timbral change is subtler but still clearly audible. Upright pianos use a different mechanism entirely.
On Beethoven's pianos, the una corda pedal shifted so far that only one string was struck per note — the timbral change was far more dramatic than on modern instruments, where two strings are still struck.