Snap hook carabiners come in four characteristic shapes:
Oval: Symmetric. Most basic and utilitarian. Smooth regular curves are gentle on equipment and allow easy repositioning of loads. Their greatest disadvantage is that a load is shared equally on both the strong solid spine and the weaker gated axis.
D: Asymmetric shape transfers the majority of the load on to the spine, the carabiner's strongest axis.
Offset-D: Variant of a D with a greater asymmetry, allowing for a wider gate opening.
Pear/HMS: Wider and rounder shape at the top than offset-D's, and typically larger. Used for belaying with a munter hitch, and with some types of belay device. The largest HMS carabiners can also be used for rappelling with a munter hitch (the size is needed to accommodate the hitch with two strands of rope). These are usually the heaviest carabiners.