The Caps are the round out side button part of the snap. Normally used with a socket (not included). The pieces sandwich the material between them and then the socket attaches to a stud or screw stud...
Grommets: Rolled Rim and Spur Washers – Nickel Plated Brass Stimpson grommets are the best available. For light duty applications. Packaged by the gross (144)