Canon Félix Kir was one of those fellows sent to earth to make the rest of us feel inadequate. Not content with being a canon, he also fought with the resistance and managed to free five thousand prisoners of war. Not content with being a priest and a war hero, he decided to become mayor of Dijon. But Canon Felix Kir was not content with simply helping the souls of his flock, the bodies of prisoners of war, and the city of Dijon. Oh no. He had to make everybody in the whole damned world happier.
How do you do that?
By adding cassis to white wine, thus making a drink called a kir. If you do it with champagne (and I don't suggest you don't), it's a kir royal.
Excuse me for a moment....
That's better. Now where was I? Ah, yes. Greek carpenters.
Greek carpenters needed measuring rods to act as rulers. These were called kanons, so canon came to mean a rule or an accepted standard. For example, the Biblical canon, which is all the books conventionally included in the Bible; and the literary canon, which is the secular equivalent. Then there's the canon, or official list, of saints that you can enter if you are canonised. And there's the rule, or canon, of a religious order under which you can live and thus become a canon like Canon Kir.
Finally, you can compose music using complicated rules. For example you can take a single theme and then repeat it backwards, upside down, at half speed or at double speed. Then you try to make those repetitions work on top of each other. It's all terribly complicated, but if you follow the canon, you will write a canon.
Here's one by Bach with an excellent explanatory video.