But the use of flags for signalling messages long remained primitive, as indicated by the 1530 instruction that when the Admiral doth shote of a pece of Ordnance, and set up his Banner of Council on Starrborde bottocke of his Shippe, everie shipps capten shall with spede go aborde the Admyrall to know his will.
Signals were primitive and rather ad hoc ("As soon as the Admiral shall loose his fore-top and fire a gun..."), and generally a one-way communication system, as only flagships carried a complete set of flags.
In 1800 Captain Sir Home Popham devised a means of extending this: signals made with a special "Telegraph" flag refererred to a separate dictionary of numbered words and phrases.
[3] A similar system was devised by Captain Marryat in 1817 "for the use of vessels employed in the merchant service".
But they retain additional flags for naval usage (such as related to maneuvering or status), and use their own codes.
It also communicates to all local parties, including personnel in the vicinity, or in passing small boats that do not have radios.