A size chart has a basis, and most charts do not tell you which

Take a skirt. Two suppliers publish "waist 72". On the first, 72 is the skirt, measured flat across the waistband with the fabric relaxed. On the second, 72 is a body, and the skirt cut from that pattern will not have a 72 waistband — it will have something smaller, because the seam allowance and the ease are already spent.

Both charts are correct. Both are useless on their own. If you measure your own waist with a tape and it reads 72, one of those two skirts fits and the other does not, and nothing on the page told you which.

So we print the basis on every chart. Where the supplier states that the measurements are garment measurements laid flat, we say exactly that. Where they state body measurements, we say that. Where they say nothing at all — and that is the majority of the charts we parse — we say they have not said, and we would rather print an honest blank than guess.

There is a trap in doing this programmatically, and it is worth naming. The obvious way to infer the basis is to look at the column names: a chart with a "Waist" column must be a body chart. It is not. "Waist", "Bust" and "Hip" are column names in almost every chart in this catalogue, including the ones that are plainly garments measured laid flat. Inferring the basis that way relabelled 345 of our 798 products as body measurements — confidently, and wrongly.

So the rule here is that a basis is declared by an explicit phrase or it is not declared at all. "Measurements" is the honest label when the supplier is silent. A mislabelled measurement is worse than a missing one, because a missing one sends you to the size chart and a wrong one sends you confidently to the wrong size.

This is the single most useful thing a size chart can tell you, and it is the thing most often missing. It costs the supplier one line of text.