A Maillon is a metal link which allows equipment to be threaded together in a more permanent manner than with a carabiner. Maillons have a threaded sleeve which tightens over a thread, as opposed to a hinged gate like a carabiner, making them stronger but more time consuming to attach and detach from the system. They offer greater versatility over carabiners as their different shapes and lack of hinged gates allow them to be used in multi-directional situations.
They are used primarily in climbing and caving systems to construct semi-permanent anchor points but can also be used for fastening harnesses with a dual attachment point.
The long opening Maillon allows easier connection of wider parts when compared to the standard opening option.