The snap shackle is a metal ring with an easily operated opening system. Complexity and security vary among the different models.
The snap shackle is used to rapidly attach two or more elements on board. It is often found on sheet and lifeline ends, etc.
Snap shackles can be either standard or quick-release, the latter fitted with a sliding pin with a small loop attached for sheet handling.
Choice of snap shackle will depend on its intended use and the load it will carry. Block and fitting catalogs provide formulas for calculating load for various tackle and lines.