超絶技巧練習曲
Well-known examples include Johann Sebastian Bach's "The Well-Tempered Clavier" and Frédéric Chopin's 24 Preludes, Op. 28. Such sets are often organized as preludes and fugues or designated as preludes or études. Some composers have restricted their sets to cover only the 12 major keys or the 12 minor keys; or only the flat keys (Franz Liszt's "Transcendental Études") or the sharp keys (Sergei Lyapunov's Op. 11 set). In yet another type, a single piece may progressively modulate through a set of tonalities, as occurs in Ludwig van Beethoven's Two Preludes through all twelve major keys, Op. 39.