Shape
The shape of a carabiner dictates what it can be used for.
When choosing a climbing carabiner, there are three criteria to consider: how you intend to use it, its shape and its locking mechanism.
How to choose a carabiner
Intended use
Non-locking carabiners
Used on the way up, they are mainly found on quickdraws or used to attach to protection and hold the climber in the event of a fall.
Locking carabiners
They have a sleeve that locks the gate onto the body of the carabiner, minimising the risk of unintentional opening. These carabiners are used for belaying, belay station installation, rappelling, securing a lanyard, and more.
D-shape
D-shape carabiners can be used for clipping in (at the end of a lanyard) or for belaying (only with a Grigri). Compared with pear-shaped models of the same diameter, D-shaped carabiners are stronger because the load is transferred along the major axis of the carabiner.
Pear shape (hms)
"HMS" carabiners can be used for belaying (with all types of belay devices or with a Munter hitch) or for clipping in (at the end of a lanyard or with a clove hitch on the rope). Their shape means the gate can open wider than other carabiners, which makes them easier to use.
Locking mechanism
There are two locking mechanisms used for locking carabiners.
Screw lock
This classic mechanism comprises a rotating sleeve on a threaded gate.
Auto lock
This type of carabiner can be opened in two or three movements of the sleeve (depending on the model). The main advantage is that the gate locks automatically, so there is no risk of forgetting to lock it.
David
Climbing product manager
Some models of carabiner, such as the Simond BLC, have a harness attachment system that always loads the carabiner along its major axis, for maximum strength.