If you compose a piece of really nice (to you anyway) music you need to write it down so that you can remember it and other people will be able to play it too. Maybe you want to orchestrate your piece? Well, you have to know all about music theory and harmony to be able to do that, otherwise a Bb clarinet will be in the wrong key, or an Eb French horn will sound terrible instead of beautiful.

If you are learning an instrument you will have to learn music theory and harmony anyway, but many people treat this vital knowledge as unnecessary and just continue to try to play the notes because they sound nice. However all music students get to a stage quite early on in their progress of learning when they suddenly find that this, to some rather boring subject, is pretty useful and so have to go about trying to learn more about it.

Rather like ones knowledge of the English language, if you know lots of unusual words for instance, it enables you to read more interesting and complicated books, so with music.

If you can get to grips with music theory you can understand so much more about the music you are learning, and in due course you may well wish to compose a piece or two of your own, write it down, let your friends play it, or even publish it if you are good enough.

The usual steps are comparatively simple after that. You learn the rules of harmony, only to break them, no doubt with your own music, and see how the great composers harmonised their melodies in different ways. You discover how to put more than one tune into a piece and create co-existing melodies which become their own harmony within existing music structures. Bachs double violin concerto is a splendid example of this with what sounds like two violins wrapping around each other to create a wonderful effect.

So having got to this stage after quite a bit of study away from your beloved instrument, be it piano, guitar, double bass or French horn you may want to take a leap into composing either for your instrument, a group of instruments or even go the whole way and hazard an orchestral piece.

This takes no little talent but fortunately many people over the years have been able to give us the wonderful music that is generically termed classical, whether it was composed in 1700 or 2000. It may be a bit tedious at first, but if you stick with it music theory will give you pleasure for the rest of your life in your appreciation of the music of all ages.