Why Do Piano Bass Strings Cross Over the Treble Strings?

Lift the lid of a grand piano and the copper-wound bass strings can be seen running diagonally above the middle and treble strings. This arrangement is called cross-stringing, or overstringing. It is not there to make the interior look complicated; it reorganises length and position within a limited case.

Low notes need relatively long, heavy strings. If every string ran parallel, as in an early straight-strung piano, the bass strings would follow one side of the case and their bridge would sit nearer the edge of the soundboard. Raising the bass strings to a second plane and stretching them along the diagonal allows a more useful bass-string length within the same outline, while moving the bass bridge towards a more central part of the board. Because piano strings radiate little sound directly and pass most of their energy through the bridge to the soundboard, string layout also determines how vibration enters the wooden plate.

The design became standard during the nineteenth century. The Smithsonian records that Steinway introduced its overstrung scale in 1855 and patented a related grand-piano design in 1859; the Metropolitan Museum likewise identifies overstringing and the cast-iron frame as influential innovations. Yet crossed strings do not automatically create a superior tone. String length, tension, bridges, soundboard and case still have to work as one system.

There is no need to listen for a mystical “crossed-string sound”. Listen instead for bass that has weight without muddiness, and for a convincing transition into the middle register. Those diagonal lines under the lid reveal that piano tone comes not only from the struck key, but also from the instrument's hidden spatial design.

https://americanhistory.si.edu/steinwaydiary/annotations/?id=2042
https://americanhistory.si.edu/collections/object/nmah_605949
https://www.metmuseum.org/art/collection