How can one tune chase itself and still sound beautiful?

A fugue is a piece in which one short tune is passed from voice to voice and keeps chasing itself. The word comes from the Latin fuga, meaning flight, and is related to verbs meaning to flee and to chase, which is a fair picture: the theme runs away, and the other voices run after it.

Here is how it goes. A fugue opens with a short main theme, the subject, sung or played by one voice alone. Then a second voice enters with the same theme, moved into a related key, usually the dominant (a fifth above) or the subdominant. This copy is called the answer. While the answer sounds, the first voice does not fall silent: it plays new material against it, and if that material is reused later it becomes the countersubject. Each voice joins in turn until all have stated the theme, and the opening section, the exposition, is complete. After that the composer is freer: the theme can return in new keys, be turned upside down, played backwards or stretched out, and in a stretto the voices pile in on top of one another before the last has finished.

Johann Sebastian Bach is famous for his fugues. He wrote 48 of them in The Well-Tempered Clavier, one in each major and minor key, which many composers and theorists regard as the greatest model of the form, and the critic Alex Ross wrote that he became a master by never ceasing to be a student. As a young organist he asked for four weeks off, stayed away for about four months, and reportedly walked 450 kilometres each way to hear Buxtehude play in Lübeck, upsetting his employer.

Because the composer decides most of the structure, a fugue is closer to a way of writing than to a fixed mould: only the opening follows a set script, and after it the composer chooses which keys, transformations and overlaps the theme goes through.