I well remember some of the notes that occurred "naturally" when I was younger! Believe me, they don't all qualify as harmonious ...

Graham, glees and catches, like rounds, have three or four parts for small groups to sing a part each. Rounds, of course, are where each singer sings the line at a different time, so that, when it's all up and running, all the lines are being sung together. Glees and catches work on similar lines, but I can never remember what exactly they are ... over to you, HK?

Steve