A press snap (four-part snap fastener, sometimes called a spring snap or “S-spring” snap) is a mechanical clutch. A sew-on button is a disc held by thread or a shank. They solve different closures. Swapping them on a tech pack because both are “round and shiny” is how a knit baby bodysuit gets a 24 mm shank button it cannot take.
Snap anatomy
A common metal snap is four pieces: cap (the visible face), socket (the spring side), stud, and post/ring (the part that clenches the cloth). The socket contains a spring — an S-spring in many apparel snaps, a ring spring in heavier coats. Plastic snaps (often POM or similar) use a different stud/socket geometry and are common in babywear because they are low-profile and come in colour.
Size names such as 655, 633 and 831 are factory series, not ISO. They tell you which cap diameters the lower three parts will accept. Always confirm millimetres and the die.
Setting is a press. Too little pressure and the snap spins or pops. Too much and you cut the knit. Stay tape or a folded facing is part of the spec on jersey.
Sew-ons
Sew-throughs and shanks need a buttonhole (or a loop). They can be removed for laundry or replaced in a tailor’s shop. They do not need a die in the garment factory, which is why small fashion brands still love them. They fail by thread abrasion and by shank breakage, not by a spring fatiguing — unless someone has used a snap-looking button that is actually a rivet.
When to use which
| Closure | Typical use | Watch-outs |
|---|---|---|
| Sew-on, 2-hole | shirts | thread shank height |
| Sew-on, shank | coats, fashion faces | shank stand-off vs cloth bulk |
| Metal snap | outerwear, leather, denim plackets | die match, corrosion, nickel |
| Plastic snap | baby knits, sports | heat, prong bite, colour crocking |
| Jeans button + rivet | denim | tack attachment, not a sew-on |
Kidswear: both snaps and buttons are small parts. Attachment force and the small-parts cylinder belong on the spec regardless of how cute the cap is.
If the designer wants a crystal face that unscrews for dry-cleaning, that is a sew-on (or a two-part screw system), not a four-part snap.