Buckle and clasp are terms used to refer to the watch closure - the part of the watch that either opens or separates allowing the watch to be worn or removed and locks in place ensuring the watch does not come off without the wearer knowing.
Watch buckles and clasps are usually made of the same metal that is used to make the watch case. There are two primary types of watch clasps: buckle and deployant/deployment clasp.
The buckle is an adjustable closure made up of a pin and a series of holes in the band. It is also referred to as a tang buckle. The tang is the pointed projection at the center of the buckle that goes into the strap hole. It works just like a leather belt buckle. It is the traditional watch clasp on leather and other non metal watch straps.
Invented in 1910 by Louis Cartier, deployant clasp, deployment clasp or deployment buckle is a folding, locking metal clasp. They are an expanding metal mechanism designed to fasten a watch band with a fitted look for a sleek and elegant look.
It connects to both sides of the watch band and closes via a folding mechanism. A folding clasp lifts out enlarging the bracelet or strap enough to slip your hand through. Metal bracelets always close by way of a deployant clasp. There are different styles and configurations of deployant watch clasps.
Outside and Inside Styles
Deployant clasp, outside style: The tail of the strap goes on the outside, just like with a traditional tang buckle. This is the more common style.
Deployant clasp, inside style: The 'tail' of the watch strap goes on the inside of the clasp providing a more elegant look. The downside is that this style may not work with just any standard strap. A custom strap is usually needed to make the tang-slot and keeper loop invisible.
A jewelry clasp is the simplest clasp. It is a latch that snaps closed around a bar. It is released by gently lifting the clasp and unsnapping it from the latch bar. It is most common on ladies dress watches.
Jewelry Clasp with Push Button
This is a jewelry clasp with a lock that is released by a push button for enhanced security.
Fold Over Watch Clasp
The fold over watch clasp collapses on itself and locks via a pressure tab. The clasp usually features several micro adjustment holes that can be used to adjust the bracelet size by non-jewelers. A weakness of fold over clasps is that the pressure tab often wears out and the clasp no longer stays closed. To solve this, here are different styles of fold over watch clasps.
Fold Over Clasp with Safety - Has a flap that folds over the closure making it more difficult to open by accident.
Fold Over Clasp with Push Button - Has a secure lock with buttons on either side of the clasp. Both buttons must be pushed to release the locking mechanism. This prevents the clasp from opening on its own without the wearer knowing and is even more secure than the Fold Over Safety Clasp.
Fold Over Clasp with Safety and Push Button or Double Locking Fold Over Clasp - A watch clasp mechanism that combines the flap of the safety clasp with a push button lock and offers the maximum security available for any fold over clasp.
Invisible Double Locking Clasp
Also known as Hidden Deployment Buckle or Butterfly Clasp, because the clasp has two “wings” that fold in toward the middle from opposite ends of the clasp so the clasp is hidden upon closure and it opens symmetrically like a butterfly by pulling the joined ends of the bracelet away from the wrist. Some have push-button locks for added security.
This is one of the most common clasps available and a customer favorite because it has the appearance of a seamless band. These clasps are also found on leather and non-metal stap watches.
Box with Tongue Clasp
The Box with Tongue Clasp has a basic locking mechanism but wears out easily making it difficult to stay locked.
Box with Tongue and Safety Clasp is a variation of the Box with Tongue Clasp with a locking bar that swings tightly onto the locking post providing additional security.
Expansion Band or Stretch Bracelet
An elastic band that expands over the wearer's wrist and then contract snuggly on the wrist when released.
Lobster Claw or Hook Buckle Clasp
This clasp looks like a lobster's claw and is typically found on chain bracelets where the claw clamps to the last link on the opposite side.
As for how to open watch clasps safely, check out this step-by-step resource on how to open and close different deployment clasps buckles.