The use of substitutes is to enable the same signal flag, either alphabetical flag or numeral pennant, to be repeated one or more times in the same group, in case only one set of flags is carried on board.
The first substitute always repeats the uppermost signal flag of that class of flags which immediately precedes the substitute.
The second substitute always repeats the second and the third substitute repeats the third signal flag, counting from the top of that class of flags which immediately precedes them. No substitute can ever be used more than once in the same group.
The answering pennant when used as a decimal point is to be disregarded in determining which substitute to use.
The signal “VV” would be made as follows:
V :- first substitute
The number “1100” would be made by numeral pennants as follows:
1 :- first substitute
0 :- third substitute
The signal “L 2330” would be made as follows:
L23 :- second substitute
In this case, the second substitute follows a numeral pennant and therefore it can only repeat the second numeral in the group.