When you fasten with a button, you simply have the button and the buttonhole. When you fasten with a snap, things can get more complicated.
Snaps are fastened by inserting a stud into a socket. The socket is sewn to the wrong side of the uppermost layer. The stud is sewn to the right side of the lowermost layer.
Snaps can be attached by hand sewing or by using a tool. If the snap is sewn by hand, there are only two parts. There is a socket and a stud.
Notice the wide variety of sizing on the snaps in the image above. Hand sewn snaps are invisible on the outside. You can’t tell that they are there.
Snaps that are attached using a snap tool have four different parts. You have the socket and the stud, but you also have attaching units on the side of the fabric opposite from the socket and stud.
The attaching unit for a socket can be a capped ring or an open ring. The attaching unit for a stud can be a post, open ring or capped post.
In the image above, the top row are the sockets, the second row are the attaching rings for the sockets, the third row are the studs and the final row is an open ring for attaching the stud. As you can see, one capped ring looks like a button!
The stud is usually applied with the post of the stud facing away from the body, which means it will be on the inner most layer of the overlap. This is so that the body doesn’t get poked with it.