Opus clavicembalisticum

Opus clavicembalisticum (Latin: "Piece for Keyboard") is a work for piano solo composed by Kaikhosru Shapurji Sorabji from 1929 through 25 June 1930. Notable for its extreme length, rhythmic and harmonic complexity and notorious difficulty, it was premièred in Glasgow by the author himself the same year.

By the time of its completion, Opus clavicembalisticum was the longest piano piece and possibly most technically demanding piano piece in existence, taking around 4–4+1⁄2 hours to play, depending on tempo. However, some works conceived by New Complexity, modernist and avant-garde composers, along with Sorabji himself, have since surpassed its statures; it is in these areas that Opus clavicembalisticum is highly regarded and primarily receives its reputation. Several of Sorabji's later works, such as the Symphonic Variations for piano (approximate duration nine hours), exceed its length.

Sorabji may have partly been inspired to compose the work after hearing Egon Petri perform Ferruccio Busoni's Fantasia contrappuntistica; to an extent, Opus clavicembalisticum is a homage to this piece. Sorabji's earlier Toccata No. 1 (1928) (likewise for piano solo and in multiple movements) exudes similar Busonian influence—in some ways prefiguring Opus clavicembalisticum.

The score, though littered with errors owing to the composer's dauntingly illegible manuscript, was published by J. Curwen and Sons in London in 1931.