A round or a canon is a musical form in which several voices or instruments perform the same material, but with staggered entries. For example, in a three-voice version of "Row, row, row your boat," some people don't get to sing the first line until others are singing "Gently down the stream," and others don't get to sing it until "Merrily, merrily, merrily, merrily." The last group to sing the opening line is the last to sing "Life is but a dream," and they sing it all by themselves. In a so-called mensuration canon, all of the voices end at the same time, which means that the later you enter the canon, the faster you have to sing – or the more you have to compress - to reach the end at the same time as everybody else. One might predict that as the canon approaches its end, its density increases arithmetically. And it does – with vertigo-producing results.