There are a great many flags and pennants, differing in color, shape, and design, each having its own particular meaning, and when three or four are shown aloft together, a number is formed, the significance of which can only be determined by referring to a code book.

Each navy has a private code, which is guarded with great care.

So particular are Governments in this respect, that the commanding officer of every ship has instructions to go to any length to destroy the code book, if capture is imminent.
During the late war with Spain it was reported at one time that the Spanish code had been secured.

This means that the Dons will be compelled to adopt an entirely new code of signals.
Besides the above systems, signalling in the navy includes various other devices.

For instance, the fog whistle can be utilized in connection with the Meyer system of numerals.