Why nuts and bolts are graded separately
Why a strong bolt in a weak nut is a weak joint.
Nuts and bolts are graded separately because the two parts fail in different ways under load, and a joint's actual strength is decided by whichever of the pair is weaker, so pairing a very strong bolt with a mismatched, weaker nut achieves nothing beyond the strength the weaker nut alone was ever going to allow.
A strong padlock on a flimsy hasp
Fitting an expensive, heavy-duty padlock to a garden shed whose hasp is held on with two thin screws shows the mismatch plainly. The padlock might resist an enormous force applied to its own body, but the door is only as secure as the weaker of the two parts holding it shut. A thief never needs to defeat the padlock at all, only to lever the hasp and its screws away from the frame, bypassing the lock's strength by attacking the part that was never upgraded to match. Buying the strongest padlock available while leaving the hasp unchanged spends money on the wrong half of the problem, and a high-grade bolt in a weaker nut does the same.
Breaking a bolt and stripping a nut
A bolt's grade describes the strength of its shank, mainly its resistance to being pulled apart in tension along its length. A nut's grade describes something related but distinct: how well its internal threads resist stripping, the threads of nut and bolt shearing past each other under load while the bolt's shank stays whole. In a bolted joint the pair fails at whichever point gives way first, so a high-grade bolt in a low-grade nut does not inherit the bolt's strength. If the nut's threads are the weaker element, they strip and let go well before the bolt comes near its own rated breaking load, and the whole margin the expensive bolt was meant to provide is wasted.
A nut's resistance to stripping comes from two things, the strength of its own metal and the number of threads it engages. A taller nut spreads the load across more turns of thread, so each turn carries less, and a thin locking nut of the same material, with only a few turns in play, strips at a far lower load than a full-height nut. Grading the nut on its own lets a standard account for both, where a single strength figure borrowed from the bolt would say nothing about either. The shed door has the same two variables in its hasp, how strong the screws are and how many of them there are, and neither shows up in the padlock's rating.
Fastener standards therefore specify matched combinations. In the common metric system the pairing is written into the markings themselves, with a bolt stamped 8.8 meant to go with a nut stamped 8, and a 10.9 bolt with a nut stamped 10. Stepping outside those pairs in the seemingly safe direction, a stronger nut than the bolt calls for, costs more and adds nothing, since the bolt was always going to be the limiting part. The reverse, an unusually weak bolt threaded into a strong nut, wastes the nut's capability in the same way.
Why the bolt is meant to break first
A matched pair is designed so that, if the joint is overloaded to failure, the bolt breaks before the nut's threads strip. That order is deliberate. A snapped bolt is an obvious, unambiguous sign that the joint has failed, visible to anyone who looks. A stripped nut can leave the bolt still sitting in its hole, looking intact, while the joint underneath has already let go, and that is a far more dangerous failure to miss during an inspection. Grading the nut a little stronger in its own mode of failure than the bolt is in its own is what arranges for the loud failure to come before the quiet one.
Mixed bins on an assembly line
A mismatched pair, drawn from two suppliers or two batches without checking that their grades belong together, is an easy risk to miss wherever more than one grade of fastener is in use. A bolt and a nut can match exactly in size, thread pitch and finish while carrying stampings for different grades that a hurried assembler overlooks, and the resulting joint can pass a casual visual check while holding far less than either part's rating suggests. So the useful check when selecting hardware is whether the bolt and nut together form one of the pairs the standard specifies, and how strong the bolt is on its own matters much less.
The risk largely disappears with a nut that is welded or otherwise permanently fixed in place, where the matching was settled at the point of manufacture. It arises wherever a bolt and a nut are paired independently, by hand or by an assembly process, from separate bins of stock, and that is why a well-run stores area keeps different grades physically apart instead of relying on a stamped marking alone to stop the wrong pair ever meeting on a workbench.