A leather wallet almost always splits along the fold rather than at a corner or seam. The failure is mechanical and predictable, and construction choices determine how long it takes.
Leather is a fiber network under strain
Leather consists of collagen fibers arranged in an interlocking three-dimensional mesh, which is what gives it strength in every direction rather than only along a grain.
Bending compresses the fibers on the inside of the curve and stretches those on the outside, and both effects concentrate along a narrow band rather than distributing across the panel.
Repeated hundreds of times a year, that cycling gradually breaks fiber bonds in the band, which is why the fold thins and eventually cracks while the rest of the piece stays intact.
The finish fails before the leather does
Most wallet leather carries a surface finish, either a pigmented coating or a wax and oil treatment, and that layer is far less flexible than the hide beneath it.
Flexing cracks the finish first, which is what produces the pale line visible along the spine of an aging wallet before any structural damage has occurred.
Once the finish is broken, the exposed leather loses moisture faster and becomes stiffer, which increases the strain on each subsequent fold and accelerates the process further.
Thickness works against durability here
A thick hide is stronger in tension but harder to bend, because the outer surface has to travel further than the inner surface around the same radius.
Makers therefore skive the leather thinner along the fold line, removing material precisely where a naive reading would say more is better.
Wallets that skip this step feel substantial when new and fail earlier, since the extra thickness translates directly into extra strain every time the wallet closes.
Contents set the bend radius
An overloaded wallet forces the spine around a tighter curve, and strain rises sharply as the radius falls rather than in proportion to it.
Cards stacked near the fold are worse than cards placed away from it, because they thicken exactly the region that has to accommodate the bend.
This is the mechanism behind the recurring advice to carry less, and it is also why slim and cardholder formats survive so much longer than traditional bifolds.
Repair is possible but limited
Conditioning restores some flexibility by replacing oils lost from the fibers, which slows further cracking without reversing damage already done.
A cracked spine can be backed with a reinforcing strip by a leather worker, though the repair is visible and the wallet never returns to its original flexibility.
The more durable answer sits in construction, since wallets built with a separate folded spine panel isolate the flexing region from the panels that carry the seams.