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Semaphore is also called an optical telegraph.
In the Semaphore alphabet, flags are held in different positions to represent different letters.
For further information on Semaphore see http://www.cimt.plymouth.ac.uk/resources/topical/semaphore/semaphore.htm
The following site will help you with the "missing word" http://en.wikipedia.org/wiki/Semaphore
(what is the missing word here?)