A lobster clasp, also known as a lobster hook, is a fastener

Fastener

A fastener is a hardware device that mechanically joins or affixes two or more objects together.Fasteners can also be used to close a container such as a bag, a box, or an envelope; or they may involve keeping together the sides of an opening of flexible material, attaching a lid to a container,...

 that is held closed by a spring

Spring (device)

A spring is an elastic object used to store mechanical energy. Springs are usually made out of spring steel. Small springs can be wound from pre-hardened stock, while larger ones are made from annealed steel and hardened after fabrication...

. The lobster clasp is opened or closed by holding a small lever, usually with a fingernail, long enough to apply, then it is attached (or removed from) a short link-chain

Chain

A chain is a sequence of connected links.Chain may also refer to:Chain may refer to:* Necklace - a jewelry which is worn around the neck* Mail , a type of armor made of interlocking chain links...

 or a ring-like structure. Lobster clasps are often used for necklace

Necklace

A necklace is an article of jewellery which is worn around the neck. Necklaces are frequently formed from a metal jewellery chain. Others are woven or manufactured from cloth using string or twine....

s, bracelet

Bracelet

A bracelet is an article of jewelry which is worn around the wrist. Bracelets can be manufactured from metal, leather, cloth, plastic or other materials and sometimes contain jewels, rocks, wood, and/or shells...

s, and keychain

Keychain

A keychain or key chain is a small chain, usually made from metal or plastic, that connects a small item to a keyring. The length of a keychain allows an item to be used more easily than if connected directly to a keyring...

s.

Lobster clasps are named as such because of their "pinch

Pinch

Pinch may refer to:* Pinch , the gripping of part of a substance, such as a powder or the skin* Pinch , a very small amount of an ingredient, typically salt or a spice* Pinch, West Virginia-Mathematics and Science:...

ing" mechanism, and they are often shaped like a lobster

Lobster

Clawed lobsters comprise a family of large marine crustaceans. Highly prized as seafood, lobsters are economically important, and are often one of the most profitable commodities in coastal areas they populate.Though several groups of crustaceans are known as lobsters, the clawed lobsters are most...

's claw.