Butterfly clasp. They are sometimes called dual-fold or tri-fold. Made out of stainless steel, these clasp are highly polished and can take straps up to 4mm thick. This is a definite must if you want to prolong the lifespan of your strap. Uses a push button mechanism to release the clasp for easy release.
Available in sizes of 16mm, 18mm, 20mm, 22mm measure at the buckle end.