Why Do the Strings Cross Inside a Modern Piano?

Open the lid of a modern grand piano and you will see copper-wound bass strings running diagonally above the steel strings of the middle and upper registers. This is not decorative. It is cross-stringing, or overstringing: the string sections occupy two planes and transmit their vibrations through bridges of different heights to the same soundboard.

Bass notes require strings that are relatively long and heavy. If every string ran broadly parallel, as in early straight-strung pianos, a sufficiently long bass section would require a longer case, while a bass bridge near the outer edge would make less effective use of the soundboard. Angling the bass strings across the others allows a greater speaking length within the same outline and places the bass bridge further into the soundboard. Smithsonian sources document several nineteenth-century makers using overstringing, with Steinway incorporating it into designs and patents during the 1850s.

What listeners hear is not the crossing itself but its consequences: more useful bass-string length within a limited instrument, stronger bass resonance and a smoother connection between registers. Admirers of straight-strung pianos may value their clearer separation of tonal regions. Cross-stringing is therefore not an absolute acoustic formula. It is a design judgement balancing case size, bass power, bridge placement and tonal blend.

https://americanhistory.si.edu/steinwaydiary/annotations/?id=2042
https://www.si.edu/object/steinway-sons-grand-piano%3Anmah_605949
https://www.govinfo.gov/content/pkg/GOVPUB-SI-PURL-gpo23127/pdf/GOVPUB-SI-PURL-gpo23127.pdf