Maritime flag signalling
Maritime flag signalling, generally flaghoist signalling, is the principal means other than radio by which ships communicate to each other or to shore; distinguished from flags showing nationality, ownership, or (for naval vessels) organizational status. Virtually all signalling by non-naval vessels (whether by flaghoist, semaphore, signal lamp, or other means) is now organized under the International Code of Signals, which specifies a standard set of flags and codes; naval vessels generally use an extended set of flags and their own codes. This article will touch on the historical development of maritime flag signalling.
Early developments 
In the Middle Ages the use of signals to communicate between ships was primitive, as seen by one admiral's instructions to his fleet in 1530:
"Whensoever, and at all tymes the Admyrall 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."
By 1653 the Royal Navy had issued instructions by which an admiral could signal various orders by hoisting flags in various locations on his ship. Modern use of naval code signalling began with the invention of maritime signal flags in the mid-17th century by the Duke of York who was created Lord High Admiral after the Restoration. A ship's message had to be approved by the Officer of the watch, and his system was augmented and changed in various ways over the following century. In 1790 Admiral Lord Howe issued a new signal book for a numerary system using numeral flags to signal a number; the number, not the mast from which the flags flew, indicated the message. Substitute flags were also instituted to indicate repeated numerals, and there was consideration of making the flags more distinct. In 1799 Captain Sir Home Popham published his first list of words and sentences which could be referenced by a number (or "code"); three subsequent editions added letter flags, with the 1801 edition numbering 2994 codes.
Marryat's Code of Signals 
Previous systems were primarily naval. The first general system of signalling for merchant vessels was Captain Frederick Marryat's A Code of Signals for the Merchant Service, published in 1817. This consisted of six parts of large numbered lists:
- A list of English Men of War.
- A list of foreign Men of War.
- A list of the English Merchant Vessels (from Lloyd's List).
- A list of Lighthouses, Ports, Headlands, Rocks, Shoals, Reefs &c.
- A selection of Sentences.
- The Vocabulary.
Different flags indicated which list was referred to. As an example, flying the Rendezvous (RE) flag (indicating Lighthouses, Ports, etc.) over the numerals 1537 indicates the ship's home port is Amsterdam. Flying Rendezvous under the number indicated the ship is sailing from Amsterdam, and flying it at some other mast-head indicates she is bound for that port. Numbers alone indicate a sentence: "4576" means "I mean to keep sail set, and carry on all night, as I am anxious to get into port." Quite a mouthful for only four flags, and expeditious. Marryat's code was an immediate success, was translated into several other languages, and because of its widespread usage the 1854 edition was renamed The Universal Code of Signals for the Mercantile Marine of All Nations. The last edition was published in 1879, two decades after the publication of the code that was to supplant it; there are reports it was still being used as late as 1890.
International Code of Signals 
Various other codes were also published, but all these were eventually supplanted by the Commercial Code of Signals published by the British Board of Trade in 1857; what eventually became the International Code of Signals (ICS). A significant development was the addition of letter flags to make the code alphabetic. (Although the vowels were initially left out to avoid formation of any objectionable words.)  During World War I there was an unprecedented need for ships, merchant as well as naval, to communicate, but the ICS was found wanting: "It was not international. It was found that when [signalling] word by word, the occasions upon which signaling failed were more numerous than when the result was successful." This led to major revisions in 1931. Additional changes in 1969 greatly reduced the Code (dropping the Geographical and Vocabulary sections), and more narrowly focused it on communications related to safety of navigation. An indication of the success of the ICS is that most navies now use the ICS flags for representing letters.
- Wilson 1986, p.77, quoting from W. G. Perrin, "British flags" (Cambridge, 1922).
- Pelham Brenton, Edward, Cptn., The naval history of Great Britain: from the year MDCCLXXXIII to MDCCCXXII., Vol. III, C. Rice, Berkeley Square, London, 1824, p.163
- Wilson 1986, pp.79–81.
- Popham, Home, Sir, Telegraphic signals or marine vocabulary 1801, C. Roworth Printer, Bell Yard, Temple Bar, (Transcribed by Peter Ball, January 2006), from signal book at the National Maritime Museum, Greenwich, UK
- The "1" (one) flag is shown with reversed colors elsewhere on the Internet. That the form shown here – white on a field of blue – is correct is confirmed by reference to Marryat 1847, Marryat 1854, and Wilson 1986.
- Marryat 1847; Marryat 1854.
- Mead 1934, excerpted at Archives & Collections Society.
- Mead 1934.
- Wilson 1986, pp.83–84; see also Hulme, Flags of the World (1898), excerpted at Archives & Collections Society.
- ICS 1931, preface.
- ICS 1969.
See also 
- The 1931 International Code of Signals (H.O. No. 102) (American ed.), U.S. Navy Hydrographic Office, 1931
- International Code of Signals (United States Edition, 1969 (Revised 2003) ed.), 1969
- Marryat, Captain Frederick (1847), A Code of Signals for the Use of Vessels Employed in the Merchant Service, by Captain Marryat, R.N. (10th ed.)
- Richardson, G. B. (1854), The Universal Code of Signals for The Mercantile Marine of All Nations, By the late Captain Marryat, R.N. (12th ed.)
- Mead, H. P. (1933), "Captain Frederick Marryat, R.N.", U.S. Naval Institute Proc. 59 (361): 371–5
- Mead, H. P. (1934), "The History of the International Code", U.S. Naval Institute Proc. 60 (378): 1083–8
- Wilson, Timothy (1986), Flags at Sea
- Excerpt from Brown's Signalling (1916), showing earliar version of ICS.
- International Code of Signals , United States edition (1969) (pdf download)
- International Code of Signals , United States edition (1969) Alternate source, courtesy of Sea Scouts.
- Sea Flags (Joseph McMillan)
- Commander H. R. Mead's article on Captain Marryat
- Captain Howeth: Historic Modes of Naval Communication
- Flags of the World: Maritime Signal Flag History