N, O, NO, NON, NOO, NOON, NONO, NONON and NONNN are the international code of signals. Each of them has a particular type of flag which has a printed message that makes it easy for identification. Here, it is also essential to mention the four NATO substitute flags, i.e. first substitute, second substitute, third substitute and fourth substitute. There are several instances and different occasion when these come into the picture while sailing.