Flag signals (and other visual forms of communication like signal lamps) were a common and important part of tactical communications (i.e. short-range ship-to-ship communications) throughout WWI, and for much of WWII. Flags had three main advantages over radio. Radio signals could be detected and intercepted by an enemy. Even if they couldn't be deciphered, these intercepted signals would allow the enemy to determine where your fleet was through detection-finding techniques. If the signals were deciphered (or sent unciphered), then this got so much worse. Flag signals could not easily be intercepted, and could not be detected outside of line-of-sight. Flag signals were much quicker than radio signals. To send a radio message, a captain would have to dictate the message, which would be taken to the signals office. It would be enciphered, turned into morse code, and transmitted. The receiving ship would have to reverse this procedure. All of these steps slowed down the sending (and receiving) of a message considerably. A flag signal could be sent and received without anyone having to leave the bridge. Finally, visual signals were, generally, more reliable than radio. Radio signals could be jammed by the enemy - or, in the early days of radio, by friendly ships sending too many messages. Flag signals could be blocked by smoke, yes, but were otherwise more reliable.

Given this, there are plenty of examples where flag signals played a significant part in battles in this period. The most famous one, though, is the Battle of Jutland, in May 1916. The early stages of the Battle of Jutland saw the British Battlecruiser Fleet (BCF), under the command of Vice Admiral David Beatty, chasing their German counterparts in the 1st Scouting Group, who were fleeing to the south. In the months before the battle, the 3rd Battlecruiser Squadron had been detached from the BCF for gunnery exercises at the main fleet base at Scapa Flow. They had been replaced by the fast battleships of the 5th Battle Squadron (5th BS), under Rear Admiral Hugh Evan-Thomas. Beatty had made little effort to integrate the 5th Battle Squadron into his command, especially when it came to communications procedures. This would turn out to be a significant problem at Jutland. The German battlecruisers were falling back towards the main body of the German High Seas Fleet, which Beatty did not know was at sea. It was only once the High Seas Fleet became visible from Beatty's flagship that he realised the danger his fleet was in; the High Seas Fleet greatly outnumbered the BCF, and could easily destroy it if he committed to an engagement. As a result, he turned away to the north, back towards the British Grand Fleet. The 5th BS, which consisted of slower ships than the rest of the BCF, was separated from the rest of the BCF, and so had to be ordered to turn. This order was passed to the 5th BS through a flag signal, reading 'Alter course in succession 16 points to starboard'. Such signals were usually executed (carried out) once the flags for the signal had been lowered. In this case, though, the flag signals were not hauled down for several minutes after they had been hoisted. As a result of this delay in hoisting this down, the 5th BS continued southwards for several more minutes. This brought them into the range of the High Seas Fleet. The 5th BS made its turn northwards under fire from the German battlefleet; fortunately, no ships were hit while turning, but several ships were struck as they moved northwards out of range. Several other significant flag signals were sent during the battle - for example, Jellicoe (commanding the Grand Fleet) ordered the Grand Fleet to deploy into line using a flag signal - but this is probably the most notorious and controversial flag signal of the battle.