Why length is taken along the top

A ruler can run along three different lines on the same shaft and produce three different lengths, none of them wrong in isolation. The published method picks one, the top, and the reason is geometric rather than arbitrary.

Three lines, one shaft

The dorsal line runs along the top surface, from the base landmark to the tip. The ventral line runs along the underside, starting further back near the scrotal junction rather than at the same base point, and following any upward curve as it goes. A lateral line, along either side, exists in principle but has no fixed edge to follow on a rounded cross-section, which makes it hard to repeat even before curvature enters into it.

These are not three measurements of the same distance taken with slightly different care. They are three different paths across the same object, and a path's length depends on where it runs.

Why ventral reads long

The underside's starting point sits closer to the body than the dorsal starting point, so the ventral path is longer from its very first centimetre, before curvature is even considered. Add an upward curve, which is common, and the ventral line runs along the outside of that curve the way the longer edge of a bent hose is the outer one - the more pronounced the curve, the larger the gap between the two lines becomes. A ventral reading is not a mistake, but it is measuring a longer path across the same shaft, and it is not comparable to a dorsal figure or to the published data, which uses dorsal throughout.

Why lateral is ambiguous

A lateral line has a subtler problem: there is no single, well-defined lateral line to be consistent about. The dorsal line runs along a clear ridge - the top centre of the shaft. The ventral line, similarly, runs along the centre of the underside. A side line has no equivalent landmark; "the side" shifts depending on where the ruler happens to sit, which makes a lateral reading hard to repeat even for the same person on two different days, independent of anything to do with curvature.

The picture

The dorsal line, solid, is the shortest and the standard. The ventral line, dashed above it, starts further back and rides the outside of the curve, which is why it is drawn longer end to end. A lateral line has no fixed position on the shaft, which is why it is shown only as a straight reference rather than a real path.

Why dorsal is the standard

The dorsal line is short relative to the other two, it starts at a landmark that is well defined - the pubic bone, reached by pressing to it as the bone-pressed protocol describes - and it stays straight along a straight shaft and follows a shorter, more consistent path than the underside on a curved one. Those three properties together are what make it reproducible from one session to the next and comparable across different people, which is the entire point of a standard method. None of this is a claim that the top is anatomically special. It is a claim that it is the most repeatable line to draw a ruler along, and repeatability is what a standard requires.

What this piece is not covering

Two related questions get their own full treatment rather than a section here. Keeping the ruler parallel to the shaft's own axis, rather than level with the floor, is a separate discipline from choosing which line to run it along - covered on its own. And how much a pronounced curve actually adds to a ventral-style reading, plus how to hold a rigid ruler as a straight chord across a curve rather than bending a tape along it, is its own detailed answer. Where the dorsal line is defined to end, at the tip, is likewise a separate question from which line runs there.

What a length along any line still is not

However carefully the line is chosen, the result is a distance in centimetres, nothing more. It is not a score, and confusing the two is common enough to be worth stating directly: a rating from an AI-judged photo on a site like Rate Cock or a human reviewer on Rate Penis is a subjective read of an image, not a path traced along a shaft with a ruler, and a photo score from Penis Rater sits in that same category rather than this one. An app that estimates a figure from a picture is doing something closer to those than to this - inferring from visual pattern rather than tracing a physical line, for reasons that are optical rather than a fixable software gap. The dorsal line, held to bone and read to the tip, is the one that produces an actual measurement.