A polyphonic composition, developed from a given theme or
   themes, according to strict contrapuntal rules. The theme is first
   given out by one voice or part, and then, while that pursues its way,
   it is repeated by another at the interval of a fifth or fourth, and so
   on, until all the parts have answered one by one, continuing their
   several melodies and interweaving them in one complex progressive
   whole, in which the theme is often lost and reappears.