The theme, or main melody, for this composition comes from a dramatic work by a much earlier composer, Henry Purcell. Purcell was one of the most famous English composers of the late seventeenth century. He was especially well known for his use of counterpoint. Counterpoint combines more than one melody at one time. Benjamin Britten uses counterpoint in the final section of The Young Person's Guide to the Orchestra .
The theme is first played by the full orchestra. Then it is played by the various families of instruments (woodwinds, brass, strings, percussion, and full orchestra again). Next, Britten shows off the individual instruments by having them play a variation of the theme. Each variation also changes the theme's dynamics (loudness), tempo (speed), and mood. There are thirteen variations: (1) flutes and piccolo, with harp accompaniment; (2) oboes; (3) clarinets; (4) bassoons; (5) violins; (6) violas; (7) cellos; (8) double basses; (9) harp; (10) horns; (11) trumpets; (12) trombones and tuba; (13) percussion. Finally, the composition ends with a new tune. It is first introduced by the piccolo and then played in a fugue (a sort of counterpoint that Britten called "a race between the instruments") by each of the other instruments of the orchestra until the brass instruments again play the main theme to close the work.