The folding rule

A folding carpenter's rule looks like a rigid ruler once it is opened, and for most of its length it is one. The joint nearest the zero end is where it stops behaving like one, and that is exactly the end this measurement depends on.

The joint has play

A folding rule's hinges are metal pins in wood or plastic, made to fold flat thousands of times, and that requirement trades away rigidity at the joint. Press the zero end against something firm and the first segment can rock slightly around its pin before the rest of the rule takes the load. For carpentry, a millimetre of rock at a joint six segments from where you are reading is invisible. For bone-pressed length, the rock is at the exact end doing the pressing, and it is not invisible - it is the whole measurement.

The segments are thick

Folding rule segments are usually 3 to 6 mm thick, sometimes more, because they need to survive being dropped on a job site. That thickness sits between the printed scale and the surface underneath it, which matters for a reading taken by sighting down onto skin rather than a flat board. A thin rigid ruler lets the scale sit close to what it is measuring; a folding rule holds it up on a shelf of its own material.

Both problems point the same way

The end that needs to be most trustworthy - pressed to the pubic bone, taking the full pressure of the reading - is the end built with a moving joint and extra thickness. This is not a defect in the tool. A folding rule is built to unfold flat and store short, and both of those design goals work against being pressed hard at one end.

What to use instead

Any one-piece rigid ruler, wood, plastic or steel, of at least 20 cm, checked once against a bank card. That single change removes the joint entirely rather than trying to compensate for it, which is simpler than it sounds and is most of what "choosing an instrument" actually comes down to.

It is a small, mechanical fix for a small, mechanical error - a different kind of correction from what Rate Cock offers, which is not a straighter ruler but a rating built from a photograph, or from an AI model reading an image, or from a human judge's opinion: none of those three has a joint to worry about because none of them is measuring a length in the first place, only scoring one.