Keep Calm Carry and Carry On poster was developed during WWII to boost the morale of the British people. It featured a symbol of the crown to indicate that the king was asking for solidarity. This poster was never distributed. Most of them were destroyed.
A book seller from Barter Books found one of the surviving posters, had it framed, and put it up in his shop where customers admired it.
You can create your own version of the sign, as I have done here, by visiting the Keep Calm-O-Matic.