The 26 letter flags, numeral pennants 0 to 9, the answering pennant and the three substitute pennants of the International Code of Signals, with practical examples from distress calls to harbour work.
How substitutes are used
A ship carries only one set of flags, so a signal with a repeated letter cannot be spelled directly. A substitute takes the place of the repeat. The first substitute repeats the first flag of the group, the second repeats the second flag, and the third repeats the third flag. A substitute is used at most once in a group, and it only repeats flags of the same class (letters or numerals) that sit directly before it.
Using flags in real situations
Today radio (VHF and DSC) is the first choice at sea. Flags are used when radio fails, in port, between ships close together, and whenever a visual signal is needed. A hoist of two or more flags reads from top to bottom. The “SOS” example above only shows how substitutes work. The flag distress signal is N over C.