Masks are magical items used by magic users in order to increase their magical ability.
Types of Masks
Various different types of masks exist, varying from masks that cover the entire head, such as Chota's, Noi's and Shin's, masks that cover the bottom half of the face, such as En's, and masks that cover the top half of the face, such as Matsumara's and Fujita's. There are also masks that don't appear to be masks at all, such as Turkey's and Shou's.
The masks are more a society element than a form to improve magic, nonetheless, depending on the quality of the mask the effects on the user change, some sorcerers buy them in stores, which is the most common way to have one, most of the normal citizens of the Magic World bought them, the ones unable to purchase one craft them from scratch, with a plain or lame look (like Shin's original mask) with no effects over their magic, mostly created to not look like a pariah in front of other sorcerers. The Magic Users with a powerful or unique type of magic go to the Devils for a specially crafted mask, they need to give them gifts for them, Beelzebub is the master of any art, and one of the best mask makers, Shin and Noi got their mask from him.
The fact that almost every single Magic User has a mask and was shown in their pages at least once, you can see every character mask in the Category: Magic-User.