Opus clavicembalisticum

Opus clavicembalisticum (Latin: "Piece for Keyboard") is a work for piano solo by English composer-pianist Kaikhosru Shapurji Sorabji, written from 1929 to 25 June 1930. Notable for its unprecedented length, rhythmic and harmonic complexity and extreme difficulty, it was premièred in Glasgow by its author in the year of its completion.

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

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 an homage to the Fantasia. 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 strewn with errors consequent of the composer's dauntingly illegible manuscript, was published by J. Curwen and Sons in London in 1931.