When the ruler cannot lie flat

A straight edge laid against a curved surface does not lie flat along it. It touches at the base and near the tip and bows away from the surface in between, and that gap is worth understanding because it changes how you have to read the tip, even though it does not change what the reading means.

What is actually happening

A rigid ruler is, by definition, a straight line. A shaft with any curvature is not. Lay the ruler along the top of a curved shaft, bone-pressed at the base, and the ruler contacts the surface at two points roughly - near the base, where you have pressed it in, and somewhere near the tip, where the curve brings the surface back up to meet the edge - with open space between them where the curve falls away from the straight edge.

This is exactly what a straight chord does against any arc: it touches the arc at its endpoints and sits inside it everywhere between. The reading you take is still correct for what the site's method actually asks for, because the literature measures a straight-line distance from bone to tip, not a distance along the curve - a tape pressed into the curve and read along its length gives a longer number that answers a different, non-standard question. The ruler lifting off the surface in the middle is not a flaw in the reading. It is what a straight chord across a curve is supposed to look like, and it confirms you are taking the measurement the standard method wants rather than the wrong one.

Where it does cost you accuracy

The gap between ruler and shaft does not change the length of the chord. It does change how easy the tip is to read, because the ruler is no longer resting flush against the surface near the point you are trying to mark. With a straight ruler and a curved shaft, the tip of the glans can sit slightly off to one side of the ruler's edge rather than directly against it, and reading straight down from above - the natural angle for most people measuring themselves - introduces a small parallax error at exactly the point precision matters most. Viewing angle when reading your own ruler covers this same parallax problem in general; a curved shaft makes it slightly worse, because the tip is no longer sitting hard against the scale the way it would on a straight shaft.

The fix is sighting square to the ruler rather than down onto the shaft: bring your eye level with the ruler's edge at the point of the tip, so you are looking along the scale rather than down through open space at an angle, and mark or read the tip from that line of sight rather than from directly above. A second pair of eyes, or bracing the ruler against something steady while you shift position to sight it properly, removes the need to hold the ruler and check your own sight line at the same time.

What this is not a case for

A curved shaft does not call for a flexible tape instead of a rigid ruler. A tape that follows the curve is measuring the arc, and the arc is longer than the chord by an amount that grows with how much curvature there is - using a tape for length covers both ways a flexible instrument reads long when it is used for a length reading. A flexible curve ruler, the kind sold for tracing arcs and shapes, has the same problem in reverse: it is built to follow a curve and report the arc's length, which is the number the published literature specifically does not use. The straight rigid ruler, lifted off the surface in the middle, is the correct tool doing exactly what it should.

The number this produces

A careful chord reading, taken with attention to the sight line at the tip, reaches the same rough accuracy any bone-pressed length reading does with a rigid ruler - on the order of a few millimetres, dominated by pressure and sighting rather than by the ruler's own precision. That is a measurement, with a defined method and a bounded error, and it is a different kind of number from a subjective judgement. A score from an image-based service like Rate Cock is not attempting a chord or an arc at all - it is a rating, not a distance - and the same distinction holds for a person's opinion on a site like Rate Penis, or an estimate an AI system pulls from a photo, which AI Penis is upfront about being recognition rather than geometry. A ranked score on Penis Rater sits in that same category: useful on its own terms, and not the chord this post is about.