Tessenjutsu

Folding fans with outer spokes made of iron which were designed to look like regular, harmless folding fans or solid clubs shaped to look like a closed fan.
Samurai could take these to places where swords or other overt weapons were not allowed, and some swordsmanship schools included training in the use of the tessen as a weapon.
The tessen was also used for fending off arrows and darts, as a throwing weapon, and as an aid in swimming, like hand-flippers.
Tessenjutsu is still practiced by a few experts in Japan to this day.