Substitute flag

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Substitute flags are part of the international maritime signal flag set. Their purpose is to allow messages with duplicate characters to be signalled without the need for multiple sets of flags.

The four NATO substitute flags are as follows:

First substitute	Second substitute	Third substitute	Fourth substitute

To illustrate their use, here are some messages and the way they would be encoded:

"N"
"O"
"NO"
"NON"
"NOO"
"NOON"
"NONO"
"NONON"
"NONNN"