For four versions our belt could not be put on

Someone looked at our renders and asked a question we had no answer to: where does the strap go when it is too long? There are no holes in it. There is no free end. There is no keeper. Every image we had published showed a closed leather loop with both ends anchored into the buckle.

It was not a detail left out of the render. It was left out of the design. A belt that cannot be adjusted cannot be put on.

Why nothing caught it

By this point the model had assertions in it. Crowned thickness against cavity depth. Module width against the gap between the guide rails. Ground contact. Wearability limits for the earpiece. All of them passed, and all of them were about parts we had already thought about.

That is the shape of the failure. A check can only fail on a question someone thought to ask, and nobody had asked how the thing gets around a waist. The belt had been a loop from the first version because a loop is what you model when you are thinking about surfaces rather than about getting dressed.

Not holes

The obvious fix is a row of punched holes, and we rejected it for two reasons that are specific to this product rather than matters of taste.

A conductive antenna runs the whole length of the strap, just under the liner. It is a metre-long loop, and being a closed loop around the torso is one of the few genuine advantages a belt has over anything worn on a wrist. Punching a row of holes through the tail cuts it.

The second reason applies to any leather belt. A hole is a stress concentration, and the tail is the part that gets bent double through a buckle thousands of times. Holes are where belts crack. Here the strap is also the only load-bearing part, because the electronics deliberately carry no tension at all, so a fatigue crack in it is not a cosmetic failure.

A ratchet instead

The free end now carries a toothed track on its inside face, and a pawl inside the buckle catches it. Twenty teeth at a 5 mm pitch, which is 103 mm of usable range. A lever under the buckle releases it.

The step size is the part worth arguing for. Punched holes are conventionally 25 mm apart, which gives about five positions and means the correct setting is usually not among them. Five millimetres is the difference between close enough and comfortable after a meal. And because the teeth face inward, none of it is visible from outside, which matters for a product whose whole argument is that it reads as an ordinary belt.

The costs, since they are real

A ratchet puts a spring-loaded pawl inside a buckle that we had previously described as purely mechanical and simple. The track has to survive tens of thousands of engagements without the teeth deforming enough to skip. The antenna still has to survive being folded through the buckle and pressed flat by the keeper, which is a bending-fatigue question rather than a cutting one, and it is not answered.

The release lever is also the same part that unlatches the buckle, which is a real usability risk: if it is not obvious which motion does which, the belt comes undone when you meant to loosen it. And a metal pawl clicks. On a product sold on discretion, that is a defect, not a feature.

The assertion that worked

Having decided the range mattered more than the length of the tail, we wrote the check against the range: at least twenty teeth, pitch no coarser than 8 mm. The first attempt produced a 66 mm track with thirteen teeth and failed. The tail looked long enough in the viewport, but a third of it is hidden inside the buckle and another slice is the tapered tip, so the toothed span was far shorter than the strap suggested.

That is the honest number to publish: the length of the toothed track, not the length of the tail. Quoting the tail would have been true and useless.

What this changes about how we work

The earlier lesson from this project was that prose does not fail, so put the numbers in assertions. This one is narrower and harder: an assertion suite is a record of what you have already thought of. It gives real confidence about those things and none at all about the rest, and the gap is invisible from inside.

One hundred and three millimetres also does not fit every waist. We expect to need more than one strap length, and we have not decided how many.