This note is intended to supplement Note 1754 and the article on pages 77 to 79 of Gazette XXVII. There is some danger that the reader of that note or article may get the impression that the writers are trying to prove some absolute truth about the signs when they are really only concerned with consequences of the definitions and conventions. There is not much difference between a definition and a convention. Often a convention is little more a an instruction about how a definition is to be interpreted. Neither a nition nor a convention can be right or wrong: it can be appropriate or appropriate.