The Well-Tempered Clavier in the context of "Melody"

Johann Sebastian Bach (31 March [O.S. 21 March] 1685 – 28 July 1750) was a German composer and musician of the late Baroque period. He is known for his prolific output across a variety of instruments and forms, including the orchestral Brandenburg Concertos; solo instrumental works such as the cello suites and sonatas and partitas for solo violin; keyboard works such as the Goldberg Variations and The Well-Tempered Clavier; organ works such as the Schübler Chorales and the Toccata and Fugue in D minor; and choral works such as the St Matthew Passion and the Mass in B minor. Since the 19th-century Bach Revival, he has been widely regarded as one of the greatest composers in the history of Western music.

The Bach family had already produced several composers when Johann Sebastian was born as the last child of a city musician, Johann Ambrosius, in Eisenach. After being orphaned at age 10, he lived for five years with his eldest brother, Johann Christoph, then continued his musical education in Lüneburg. In 1703 he returned to Thuringia, working as a musician for Protestant churches in Arnstadt and Mühlhausen. Around that time he also visited for longer periods the courts in Weimar, where he expanded his organ repertory, and the reformed court at Köthen, where he was mostly engaged with chamber music. By 1723 he was hired as Thomaskantor (cantor with related duties at St Thomas School) in Leipzig. There he composed music for the principal Lutheran churches of the city and Leipzig University's student ensemble, Collegium Musicum. In 1726 he began publishing his organ and other keyboard music. In Leipzig, as had happened during some of his earlier positions, he had difficult relations with his employer. This situation was somewhat remedied when his sovereign, Augustus III of Poland, granted him the title of court composer of the Elector of Saxony in 1736. In the last decades of his life, Bach reworked and extended many of his earlier compositions. He died due to complications following eye surgery in 1750 at the age of 65. Four of his twenty children, Wilhelm Friedemann, Carl Philipp Emanuel, Johann Christoph Friedrich, and Johann Christian, became composers.

A prelude (German: Präludium or Vorspiel; Latin: praeludium; French: prélude; Italian: preludio) is a short piece of music, the form of which may vary from piece to piece. While, during the Baroque era, for example, it may have served as an introduction to succeeding movements of a work that were usually longer and more complex, it may also have been a stand-alone piece of work during the Romantic era. It generally features a small number of rhythmic and melodic motifs that recur through the piece. Stylistically, the prelude is improvisatory in nature. The term may also refer to an overture, particularly to those seen in an opera or an oratorio.

A prelude and fugue generally consists of two movements in the same key for solo keyboard. In classical music, the combination of prelude and fugue is one with a long history. Many composers have written works of this kind. The use of this format is generally inspired by Johann Sebastian Bach's two books of preludes and fugues — The Well-Tempered Clavier — completed in 1722 and 1742 respectively. Bach, however, was not the first to compose such a set: Johann Caspar Ferdinand Fischer wrote a 20-key cycle in his 1702 work Ariadne musica.

There is a long tradition in classical music of writing music in sets of pieces that cover all the major and minor keys of the chromatic scale. These sets typically consist of 24 pieces, one for each of the major and minor keys (sets that comprise all the enharmonic variants include 30 pieces).

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 2 Preludes through all the Major Keys, Op. 39.