Excerpt from Manual of Visual Signaling of the U. S. Signal Corps

So it may be said that visual signals are either tran sient or permanent - transient when each signal disap~ pears as soon as made; permanent when the signal remains for a time in view, as when ﬂags are kept hoisted to be read, or a symbol is retained in a certain position. All systems of signals are based upon the principle that having a certain number of elementary signals, each in some way distinguishable from the other, they are made to appear singly or together in all possible arrangements and combinations necessary to form other signals.

It may be necessary to know the number of signals that can be made with any given number of available elementary signals where the elementary signal is used only once in each display or where it may be used repeatedly up to the given number of places in the signal which it is desired to display. Under these conditions the possible number of different signals may be determined from the rules for the equation of arrangements.

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