In the spring of 1939, the British government commissioned three posters with just the crown of King George the 6th as a graphic device. They were intended to go up in shop windows and inspire the British people during the war. The three final designs read as follows:
Your courage, your cheerfulness, your resolution, will bring us victory.
Freedom is in peril, defend it with all your might.
Keep calm, and carry on.
The third message was for a time of invasion, and was never officially used. It was rediscovered 50 years later. Here is an interesting, short video on the story: