Carabiner Shapes Explained: D vs Offset-D vs Oval vs HMS Pear

Carabiner shapes fall into four families – D, offset-D, oval and HMS (pear) – and each shape decides where the load sits. A D-shaped carabiner is the strongest for its weight because its geometry drives the load toward the solid spine; an offset-D (modified D) gives up a little strength for a wider gate opening and lower weight; an oval shares the load between spine and gate, so it is weaker but keeps pulleys and devices centred; and an HMS or pear carabiner has a wide, symmetrical top for Munter hitch belays and several connections at an anchor. Shape also sets the certification target: EN 12275:2013 requires 20 kN on the major axis for basic (Type B) and HMS (Type H) carabiners, but 18 kN for ovals (Type X).

Why does carabiner shape change strength?

A carabiner is a curved loop, so any load that sits away from the spine bends the frame instead of pulling it in tension. In a D the rope slides into the narrow end, close to the spine’s line; in an oval the symmetrical ends hold it on the centre line, so the gate, hinge pin and nose carry a bigger share. DMM gives 23–30 kN as a typical range for alloy connectors, and two widely sold current ovals are rated 23 and 25 kN – above the Type X floor, but reached with more metal. Other geometry matters too:

• Gate-open strength. With the gate open the frame becomes a hook; published gate-open ratings on aluminium lockers typically sit between 6 and 9 kN. Wider gates mean longer unsupported spans, so big-opening designs need heavier sections.
• Minor-axis loading. Loaded across the gate, the frame bends in its weakest direction. Petzl’s published examples for specific models put minor-axis strength at about 35 percent of major-axis strength, and loading over an edge at about 30 percent.
• Three-way loading. Three strands pulling in different directions push load toward the gate; Petzl notes that the strength loss depends on the angle between them.
• Nose design. A keylock nose seats into a slot in the gate and does not snag hangers, slings or harness loops. A notched hook nose snags more readily, and a carabiner caught on its nose can fail far below its rating.
• Cross-section. Forged I-beam sections put metal where bending stress peaks and save weight at the same rating. Rope-bearing surfaces need a generous radius; the informative annex of UIAA 121 illustrates a profile at least 8 mm wide with a minimum 4.5 mm radius.

D vs offset-D vs oval vs HMS: what is each carabiner shape for?

Choose by job: D for strength per gram, offset-D for easy clipping, oval for centred devices, HMS for belays and anchors.

• D (symmetrical D). Strongest per gram and simplest to load correctly: locking connectors for rescue and rigging, gear attachment and most steel industrial connectors. The trade-off is a smaller gate opening.
• Offset-D (modified D). The nose end is widened to open the gate further while keeping the load near the spine. The default for quickdraws, racking and many lockers, slightly weaker than a pure D of the same section.
• Oval. Symmetrical, so connections self-centre when weighted: pulleys, rope clamps, aid climbing, racking nuts, and rope-access and tree-care systems.
• HMS (pear). A wide, rounded top gives a Munter hitch room to flip and lets a rope, belay device and slings share the basket. Almost always locking, usually larger and heavier than a D.

Which carabiner shape is best for anchor work?

For climbing and rope-rescue anchors, use a locking HMS (pear) carabiner at the master point, where several connections or a Munter belay meet, and locking D or oval connectors where single-direction strength or a centred device matters. For industrial fall-arrest anchorages, use a self-closing, self-locking connector rated to ANSI/ASSP Z359.12 or EN 362, most often a steel D or offset-D.

The wide HMS top keeps rope and sling strands side by side rather than stacked against the gate.

Fall-arrest anchorage is about gate strength as much as major-axis strength. ANSI/ASSP Z359.12-2019 requires 5,000 lbf (22.2 kN) along the major axis for one minute and 3,600 lbf (16 kN) on the gate face, gate side and minor axis, because at a beam strap or anchor D-ring the gate can be pressed against steel; the 2019 edition added a transverse dynamic test for gate openings larger than 1 in. OSHA’s 29 CFR 1910.140 mirrors the 5,000 lb and 3,600 lb gate figures.

Whatever the shape, load along the spine, keep the gate away from rock or steelwork, never load over an edge, and use a captive pin wherever a connector could rotate into a cross-load.

Which standards and classes apply to each carabiner shape?

Climbing standards map shape onto a type; fall-protection standards classify by function, so shape only decides which class is realistic.

• EN 12275:2013. Type B (basic) 20 kN major axis / 7 kN gate open / 7 kN minor; Type H (HMS) 20 / 6 / 7 kN; Type X (oval) 18 / 5 / 7 kN; further types K (via ferrata), A (anchor), T (directional) and Q (screwlink). The gate-open test is waived where the gate locks automatically, and H, K and X carabiners carry their type letter.
• UIAA 121 (version 4, 2018). All of EN 12275:2013 plus an 8 kN edge test for Type K; our UIAA certification guide explains the label.
• EN 362:2004. Classes B, M (multi-use), T (termination), A (anchor) and Q (screwlink). B, M, T and A need 20 kN locked and 15 kN unlocked on the major axis; minor-axis minimums are 7 kN for B and 15 kN for M; Q needs 25 kN major and 10 kN minor. A self-locking gate needs at least two consecutive deliberate actions to open.
• ANSI/ASSP Z359.12-2019. 5,000 lbf major axis, 3,600 lbf gate and minor axis, self-closing and self-locking gates only.

Geometry explains the gaps. Type X minimums are lower because a symmetrical frame cannot load the spine, though well-proportioned forged ovals can still meet Type B. Class M’s 15 kN minor axis is hard to reach with slender aluminium, which is why multi-use connectors are more often steel. The US 3,600 lbf gate compares with EN gate-face and gate-side checks of 1 kN and 1.5 kN, which is why ANSI connectors carry heavier gates. Our guide to kN ratings explains the three figures on the spine.

Screwgate, twist-lock or triple-action: which lock suits each shape?

A screwgate is locked by hand – light and common on HMS belay carabiners, but reliant on the user. Twist-lock and triple-action gates relock themselves and open only with two or three consecutive actions, suiting rescue, work positioning and US fall arrest; our snap hook types guide compares them.

How is each carabiner shape manufactured?

Every shape and size needs its own forging die set. Aluminium bar, usually 7075, is cut, bent into a rough C so metal sits near its final position, and hot forged in a closed die that sets the outline, section, gate clearance and nose. The flash is trimmed, the nose and gate pin bore are CNC machined, and the body is solution treated and aged to the T6 temper, then anodised. Steel connectors are forged, heat treated and zinc plated or galvanised; our OEM carabiner materials article covers the aluminium-versus-steel decision.

One forged body can take several gates – solid, wire, screw, twist-lock or triple-action – so a range is planned around body dies, not SKUs. Type examination pulls samples to destruction on every axis the standard names; in production, batch samples confirm the minimums, and US fall-arrest connectors are commonly proof loaded unit by unit to the 3,600 lb figure in OSHA’s rule.

What should a brand specify for a private-label carabiner range?

• Shape and size: D, offset-D, oval or HMS, length and width, and intended use.
• Strength targets: major-axis, gate-open and minor-axis figures, set above the class minimum.
• Gate type and opening: straight, bent or wire, screwgate, twist-lock or triple-action, and clear opening in millimetres.
• Nose type: keylock or notched, and any captive pin for lanyard terminations.
• Cross-section: I-beam for weight, or a round rope-bearing radius for belay and lowering.
• Finish and colour: anodise colour or hard anodising, plating for steel, colour coding.
• Standards and markets: EN 12275 type, EN 362 class, UIAA 121, ANSI/ASSP Z359.12, per SKU and market.
• Marking content: brand name, kN values, type letter or standard and class, CE mark where applicable, batch code.
• Packaging and instructions: retail card or bulk, instruction languages and required warnings.

Working with Power Honour

Power Honour forges, machines, heat treats and anodises connector bodies in-house in Taiwan, and our catalogue covers aluminium and steel carabiners, including ANSI carabiners. Send a drawing, sample or target specification with the standards you need, and our engineers will review it for manufacturability and reply within two business days.