Mario Cap
Mario's Hat is an important part of Mario's clothing. His hat has an red M symbol. This hat has appeared in every game with Mario (Except the Dr. Mario series). Mario has worn this hat since the first day of his life!
The first game where it was important was Super Mario 64. There were many ways Mario could lose his hat, among them having it stolen by a Ukiki, and when he did lose it, he took more damage. The same happened in Super Mario Sunshine with an added effect of gradually giving him damage. The same effects are presumed to occur again in upcoming Super Mario Galaxy.
By taking advantage of certain glitches in the game, Mario could end up with more than one hat in Super Mario 64. Mario could use the duplicate hat as an effective weapon.
A few Badges in Paper Mario: The Thousand-Year Door can change his hat as well as his shirt. This can be done by equipping the L Emblem Badge (which gives him Luigi's clothes), the W Emblem Badge (which gives him Wario's clothes), or a combination of both.(which gives him Waluigi's clothes) Both use 0 BP and have no effects other than changing his appearance.
