ANSI Z359 defines a snaphook as “a connector comprised of a hook-shaped body with a normally closed gate or similar arrangement that may be opened to permit the hook to receive an object and, when released, automatically closes to retain the object.” 

Snaphooks typically “snap” closed and generally come in two types:    

1. Automatic-locking: These hooks have a self-closing and self-locking gate which remains closed and locked until intentionally unlocked and opened for connection or disconnection. Required by ANSI Z359.
2. Non-locking: These hooks have a self-closing gate which remains closed but not locked until intentionally opened for connection or disconnection. Not permitted by ANSI Z359.

A double-locking snaphook is an automatic-locking hook. The “double” refers to the two-step action required to unlock and open the gate. It also refers to the self-closing and self-locking operations that take place once the hook is released.