Grommets are used to reinforce holes in leather and fabrics. They can be made of metal or plastic, and are easily used in common projects, requiring only the grommet itself, a grommet-setting tool (a metal rod with a convex tip which usually comes with the grommets when you buy them in a store), and a hammer. Fancier grommet presses exist but generally a hammer and the grommet-setting tool works at least as well for setting just a few grommets.