Canon (music)
Probably the greatest writer of canons in the eighteenth century was Johann Sebastian Bach.  Many of his organ works have canons.  He wrote a famous set of Canonic Variations on "Vom Himmel hoch da komm' ich her" which have several canons at different intervals and inversions.  He probably  wrote this to show young composers how to write good canons.  Bach also wrote a work called the Musical Offering'' which has what he calls a “canon per augmentationem contrario motu” (canon in augmentation and contrary motion i.e. backwards) as well as a “canon per tonos”.  This last one is a modulating canon which means that the tune changes key.  This is hard to compose well so that it sounds good because when the first voice has just changed key the other one is still catching up in the other key.