Why Does the Una Corda Pedal Change Colour, Not Just Volume?

The left pedal of a modern grand piano is usually called una corda, meaning “one string”. When it is depressed, the keyboard and action shift sideways, so the hammers strike fewer strings or make contact with a less-used part of the hammer felt. A Broadwood grand from about 1820 in the Metropolitan Museum of Art can be adjusted so that its hammers strike one or two strings instead of the usual three. The term therefore began as a description of a physical change in how the strings were struck.

On a modern grand, not every note literally changes from three strings to one, so the pedal should not be treated as a simple volume switch. It alters the point of contact, the edge of the attack and the way multiple strings vibrate together. The sound may become thinner, more distant or more muted. This is primarily a change in tonal colour; reduced loudness is only one consequence.

Its musical value is not that it rescues an uncontrolled pianissimo, but that it can create another degree of distance when the phrase requires it. If the fingers cannot shape soft playing, the pedal may make the sound quieter without giving it direction. Added to an already controlled touch, however, it can make a repeated phrase seem to return from a different space. When Beethoven marked una corda, due corde and tre corde, he was asking for a graded change in string participation. The important question is therefore when the colour changes and whether that change serves the phrase, not whether the pedal remains fully depressed.

https://www.metmuseum.org/art/collection/search/503056
https://www.metmuseum.org/essays/the-piano-the-pianofortes-of-bartolomeo-cristofori-1655-1731