In the Font Info under the Instances tab, underneath the style name, there are the Weight and Width option menus with values behind it: Thin is 100, Heavy is 900, Ultra Condensed is 1, Ultra Expanded is 9. Being the OCD-er that I am, I’m wondering: “How wide is Ultra Expanded or Condensed? How bold is regular?”
Are there some guidelines for that? I know that the weight values 100-900 are a thing in CSS. Did the W3C who standardized CSS also specify how thick the stems should be when the weight is 300? Or did they measure the average greyness of the font? Likewise, can we say that Condensed (3) is 75% of the width of Medium (5)?
For weight, there are only the words, ‘Light’, ‘Regular’, etc. and the ID numbers that correspond to them, 100, 200, 300, etc. The actual stem thickness is not standardised though. Some type designers will tell you theories how stem thicknesses could/should be distributed. You can get a glimpse of the currently popular theories in the mekkablue script Interpolation > Insert Instances. Look for explanations in the tooltips.
There is, however, such a context for the width: See the explanation in the specification for OS/2 usWidthClass:
Although every glyph in a font may have a different numeric aspect ratio, each glyph in a font of normal width is considered to have a relative aspect ratio of one. When a new type style is created of a different width class (either by a font designer or by some automated means) the relative aspect ratio of the characters in the new font is some percentage greater or less than those same characters in the normal font — it is this difference that this parameter specifies.
The valid usWidthClass values are shown in the following table. Note that the usWidthClass values are related to but distinct from the scale for the ‘wdth’ axis that is used in the ‘fvar’ table of variable fonts and in the STAT table. The “% of normal” column in the following table provides a mapping from usWidthClass values 1 – 9 to ‘wdth’ values.
||% of normal