The Z flag is one of the international maritime signal flags.
The Z flag (upper left) is still flown daily from the signal yardarm of Mikasa, now a museum ship
Members of Shuken Kaifuku wo Mezasu Kai bearing the Z flag and the Rising Sun flag
International maritime signal flags
International maritime signal flags are various flags used to communicate with ships. The principal system of flags and associated codes is the International Code of Signals. Various navies have flag systems with additional flags and codes, and other flags are used in special uses, or have historical significance.
Two sailing ships dressed overall with their signal flags
Sailor prepares signal flags for operations at sea