The Tessen was a folding fan with an iron frame that when folded, was used to club an enemy. A few different variations were invented, including one with a sharp edge on the outside of the framework so that, instead of only being able to club a person, the user could cut as well. The Tessen was considered a mark of authority and was designed to look like your ordinary harmless fan.
The Shoninki mentions using a fan placed on the end of the sword to tell whether a far away person with a lantern is going towards the shinobi, or away from him. This would be done by ducking down, holding the sword upright, and placing the folded fan horizontally on the sword in a "T" shape. If the person is going away from the shinobi, his lantern's light will move upwards above the fan. If he is going towards the shinobi, his light will go downwards. This technique also allows a person to know if another is going to the left or right.