Johann Sebastian Bach

It is said to set down on paper a young organist's bold improvisation, as though he were testing the voice of the new organ he played in his Arnstadt days.

• Opening with a short descending figure (motif) that rings out like thunder, it seizes the audience from the very first bar.
• The toccata (an improvisatory piece played as if letting the fingers run free) is famous for the passage in which the organ pedals roll out the low notes.
• The fugue that follows (a piece in which a single subject enters in turn in several voices and builds up) grows majestic as its short subject piles up, layer upon layer, from voice to voice.
• It is often used in film and on stage as the signature music for 'scary scenes', yet whether Bach actually wrote it himself remains a matter of debate among scholars today.

From Köthen's golden age of secular music, it is one of six concertos dedicated to the Margrave of Brandenburg, brilliantly woven as if to show off the skill of the court orchestra.

• The heart of the piece is its texture: three violins, three violas and three cellos each divided into independent parts, so that nine players trade lines as if in conversation.
• The first movement sustains its energy throughout with imitation (canon), in which several voices repeat the same rhythm as if chasing one another.
• The slow second movement has only two chords written in the score, and the custom of performers improvising to fill the space between them continues to this day.
• The third-movement finale, at a fast tempo, has the voices trading phrases as if running up and down stairs, giving the thrill of a chamber ensemble moving as one body.

A vast undertaking written in Köthen for his children's education and for keyboard practice, pairing a prelude and fugue in every major and minor key, it went on to become the textbook of keyboard music.

• The first piece, the Prelude in C major, consists entirely of broken chords (arpeggios) that let the harmony flow out one note at a time, revealing the beauty within simplicity.
• Covering all twenty-four keys was itself radical for the time. In an era when keyboard tuning had not yet been settled, it carried the intention of proving that every key could be used.
• Each prelude-fugue pair has a sharply distinct character, and enthusiasts often part ways over which key they love best.
• If you follow by eye, as you listen, the order in which each fugue's subject passes between the voices, the texture sounds as intricately built as a work of architecture.

A radical form for cello alone, without accompaniment, it was presumably written with an outstanding cellist of the Köthen court in mind.

• The prelude's opening bars begin low and rise and fall like a wave, a single melodic line that implies the whole harmony: a single cello conjures the resonance of many voices.
• Each suite follows the dance sequence of its day, allemande, courante, sarabande and the rest, and among devotees the slow, heavy rhythm of the sarabande is counted the emotional summit.
• The story of how the suites were long dismissed as mere études until the twentieth-century cellist Pablo Casals restored them to the concert stage is still retold today.
• Because harmony and melody must sound at once with no accompaniment, performers differ widely in which notes they sustain and which they cut short.

The summit of the church music he was obliged to produce year after year as Thomaskantor, this monumental work renders the Passion narrative of St. Matthew's Gospel in three dimensions through chorus, soloists and orchestra.

• The opening chorus depicts a scene of lamentation in which two choirs and orchestras call and answer each other, and the crowning touch is the layered structure in which a boys' choir sings a chorale (a hymn melody sung by the congregation) on top of it all.
• Jesus's words are always accompanied by the strings like a soft halo, which vanishes only at the cry from the cross, 'My God, why have you forsaken me' — a deliberate design, it is understood, shaped to the text.
• In the alto aria 'Erbarme dich', sung after Peter has denied Jesus three times, the sobbing line of the solo violin gives voice to the feeling of repentance.
• The final chorus, 'Wir setzen uns mit Tränen nieder', closes the whole work with quietly settling harmonies; at nearly three hours long, the piece is experienced both as a religious rite and as a great epic.

Tradition holds that it was commissioned for Count Keyserlingk, who suffered from insomnia, but in fact it was an independent publication intended as a summation of keyboard music.

• The slow aria (a melody sung as if in song) that appears identically at the beginning and the end serves as a frame enclosing the thirty variations, and when the melody returns unchanged at the close, it brings a sense of a long journey coming to rest.
• Every third variation is a canon (a technique in which the same melody is taken up by another voice after a delay), and hidden within is an intricate design in which the interval between the voices widens by one step as the work proceeds.
• Variation 25 drifts through unusually chromatic, dark harmonies, and performers call it by the nickname 'black pearl'.
• Variation 30 is a quodlibet (a playful technique of weaving several melodies together) that layers two folk tunes popular in its day, tossing a laugh at the end of a serious journey.

In Potsdam he improvised on a difficult theme that Frederick the Great tossed him on the spot, then returned to Leipzig, completed it as an intricate contrapuntal collection, and dedicated it to the king.

• The theme the king gave (the royal theme) is twisted with chromaticism and fiendishly hard to treat as a fugue, yet Bach worked it out on the spot as a 3-voice ricercar (a strictly contrapuntal piece reviving an older style) and later expanded it to 6 voices.
• The passage known as the 'crab canon' is built so that the music is the same even when the score is read backward, and performers and scholars still delight in solving its puzzle.
• The endlessly rising canon is designed to climb a whole tone (one step) with each turn through the keys, so that after six repetitions it arrives an octave higher, turning into music the illusion of an endless ascent.
• The movement of the trio sonata with flute is counted the most songlike and relaxed passage amid the learned counterpoint.

The last great work, in which a single simple theme is put through every contrapuntal transformation possible, left unfinished at his death.

• The simple fugues at the start alternate the same theme in its upright and inverted forms (the melody turned upside down), inviting the listener to compare how a single seed grows into different shapes.
• As the work proceeds the theme is clothed in ever more complex garments, until the multiple fugues of the later part form a structure in which several themes intertwine and pile up at once.
• In the manuscript, the last piece introduces as a theme the note names of Bach's name (B-A-C-H, in German note names equivalent to B♭-A-C-B), and then the score breaks off abruptly. His son Carl Philipp Emanuel wrote at this point, 'Here the composer died.'
• Because the score gives no instruction as to which instruments should play it, it is performed in a wide range of scorings, from organ and keyboard to string ensemble, each bringing its own color.