kvadrantCyrillic

Source Sans 3, instanced on its wght axis once per Metro weight.

Five entries, not one, and that was a defect rather than a refinement. The family used to hold a single font at 370, so Compose matched it for every requested weight and the axis stayed where it was: a SemiBold heading rendered Latin bold and its Cyrillic at 370 beside it, visibly lighter. Cyrillic could not be made bold at all.

The axis values are measured, not derived. Ink coverage — lit pixels over the area of the drawn line — is compared between Selawik at the Metro weight and Source Sans at each candidate, and the closest wins. The method's own control is that it rediscovers 370 for SemiLight, which B-03 had found by eye; it does, exactly, which is why the other four are trusted.

They do not sit on a straight line and there is no offset that produces them — Source Sans runs relatively heavier than Selawik at the thin end and lighter at the thick end, so +130 at Light becomes −10 at Bold. Any rule fitted to one weight would have been wrong at the other, which is the argument for measuring five times rather than once. InkParityTest holds them.

The companion at one weight, for a caller that wants a single instance — the fitting fixtures do, and so does anything comparing two axis values side by side.