Johann Sebastian Bach

It is said to be a piece in which the young organist's bold improvisations, as if testing the resonance of the new organ he played in his Arnstadt years, were set down on paper.

• The opening's brief descending figure (motif) rings out like thunder, seizing the audience from the very first bar.
• In the toccata (an improvisatory piece played as if freely limbering the fingers), the passage where the organ pedals roll out low notes is especially famous.
• The fugue that follows (a piece in which a single subject enters successively in different voices and accumulates) grows majestic as its brief subject piles up layer upon layer across the voices.
• It is often used in film and performance as emblematic music for 'frightening scenes,' but whether Bach actually wrote the piece himself remains a subject of scholarly debate today.

One of six concertos dedicated to the Margrave of Brandenburg from the golden age of secular music at Köthen, brilliantly crafted as if to show off the court orchestra's skill.

• Three violins, three violas, and three cellos are each divided into independent parts, and the texture in which the nine players trade phrases as if in conversation is the heart of this piece.
• The first movement sustains its energy throughout with imitative (canonic) writing, in which the same rhythm is chased and repeated from one voice to another.
• The slow second movement consists of only two chords written on the page, and the tradition of performers improvising to fill the space between them continues to this day.
• The third-movement finale moves at a fast tempo, with the voices passing phrases back and forth like footsteps climbing and descending a staircase, giving the pleasure of an ensemble moving as one body.

A vast undertaking composed at Köthen for the education of his children and keyboard practice, pairing a prelude and fugue in every major and minor key, it later became a textbook of keyboard music.

• The opening Prelude in C major consists entirely of broken chords (arpeggios) that unfold each note of a chord in a flowing line, revealing beauty within simplicity.
• The very fact that it covered all twenty-four keys was, for its time, a radical departure. At a period when keyboard tuning systems had not yet been standardized, it carried the intent of proving that every key could be used.
• Each prelude-and-fugue pair has a distinctly different character, and enthusiasts often find their tastes divided over which key they prefer.
• If one follows, with eye and ear, the order in which each fugue's subject moves between voices, the texture sounds as intricate as a piece of architecture.

An unconventional form for solo cello without accompaniment, thought to have been written with an outstanding cellist at the Köthen court in mind.

• The overture's opening bars begin on a low note and rise and fall like a wave, a single melodic line implying the harmony as a whole — a passage where one cello alone conjures the resonance of polyphony.
• Each suite follows the same sequence of period dance forms — allemande, courante, sarabande — and among enthusiasts the slow, weighty rhythm of the sarabande movement is regarded as the emotional peak.
• For a long time it was treated merely as a study piece, and the story of how the twentieth-century cellist Pablo Casals revived it for the concert stage is still told today.
• Because it must convey both harmony and melody at once without accompaniment, performers' interpretations differ widely on which notes to sustain and which to clip short.

The pinnacle of the church music he was required to produce every year as Thomaskantor, a monumental work that renders the Passion narrative from the Gospel of Matthew in three dimensions through chorus, solo voice, and orchestra.

• The opening chorus depicts a scene of lamentation, with two choirs and orchestras calling and answering one another, and the layered structure reaches its height as a boys' choir sings a chorale (a hymn melody the congregation once sang together) over the top.
• Jesus's words are always accompanied by strings that trail after them like a soft halo — except at the moment on the cross when he cries, 'My God, why hast thou forsaken me,' when that halo vanishes, a deliberate design known to match the text.
• In the alto aria 'Erbarme dich' ('Have mercy'), sung after Peter denies Jesus three times, a solo violin's weeping melody sings the feeling of repentance in his place.
• The final chorus, 'We sit down in tears,' closes the whole work with harmonies that settle quietly, and with a performance time reaching three hours, it is experienced as both a religious rite and a single great narrative.

According to legend it was commissioned for the insomnia-plagued Count Kaiserling, but in fact it was an independent publication aimed at summing up the whole art of keyboard music.

• A slow aria (a singing melodic line) appears identically at the beginning and end, framing the thirty variations, and when this melody returns unchanged at the close, it leaves the feeling of a long journey coming to an end.
• A canon (a technique in which the same melody is echoed after a time lag) appears once every three variations, and hidden within is an intricate design in which that time lag widens by one interval as the piece progresses.
• Variation 25 flows through unusually chromatic, dark harmonies and is nicknamed the 'Black Pearl' among performers.
• Variation 30 is a quodlibet (a playful technique of weaving several melodies together) layering two folk tunes popular in his day, a moment that offers a laugh at the end of a serious journey.

After improvising in Potsdam on a fiendishly difficult theme Frederick the Great posed to him on the spot, he returned to Leipzig and completed it as an elaborate collection of counterpoint, dedicating it to the king.

• The theme the king gave him (the 'royal theme') is twisted with chromaticism, making it extremely difficult to treat as a fugue; Bach worked it out on the spot as a three-part ricercar (a strict contrapuntal piece reviving an old style) and later expanded it into six parts.
• The passage known as the 'crab canon' is constructed so that the score reads as the same music even backward, and performers and scholars still enjoy puzzling it out today.
• The 'canon per tonos,' or endlessly rising canon, is designed to climb a half step higher each time it completes a full cycle of keys, giving musical form to the illusion of endless ascent.
• The trio sonata movement featuring flute is regarded as the most song-like and relaxed passage amid the work's scholarly counterpoint.

His final monumental work, testing every possible contrapuntal variation on a single simple theme, left unfinished by his death.

• The simple fugues at the outset alternate the same theme in its original and inverted forms (the melody flipped upside down), offering the pleasure of comparing how a single seed grows into different shapes.
• As the piece proceeds, the theme dresses itself in ever more complex garb, and the multiple fugues of the later sections form a structure in which several themes intertwine and accumulate at once.
• In the manuscript, the final piece introduces as its theme the musical spelling of Bach's name (B-A-C-H, corresponding in German note names to B flat–A–C–B natural), and the score then breaks off suddenly. At this point his son Carl Philipp Emanuel wrote, 'Over this fugue, the composer died.'
• The score gives no instruction as to instrumentation, so it is performed in arrangements ranging from organ and keyboard to string ensemble, each lending it a different color.