Named instance

A variable font is a continuum. Along its variation axis for weight every value from 100 to 900 exists and is drawable, which is a lot of freedom and no vocabulary at all. So the designer plants flags: this point is Light, this one is Regular, this one Semibold, this one Black. Each flag is a named instance, a coordinate in the design space plus a name for it, recorded in the font’s own tables. Named instances are what a font menu lists, what a design tool’s style dropdown offers, and what @font-face rules and CSS keywords resolve to. They are also what a static export produces: pick an instance, cut it out of the variable file, and you have the single-weight font files the family used to ship as.

The important thing about the flags is how few there are. A weight axis running 100 to 900 holds 801 integer positions, and a typical family names nine of them. Everything between the flags is real, renderable type that no name reaches. Ask for font-variation-settings: 'wght' 520 and the font answers with a drawing that sits between Medium and Semibold, and there is nothing to call it. That gap is not a defect. It is the whole reason the format exists: a headline can be tuned until it optically matches the paragraph under it, and a weight can be interpolated during an animation, neither of which a list of nine files can do.

The names are worth respecting anyway, for two reasons. They are the family’s own editorial judgment about which points are useful, usually spaced so that adjacent steps are distinguishable at text sizes rather than evenly spaced in axis units. And they are the interoperable currency: a named instance survives a handoff, an export, a font menu, and a colleague’s machine, while 'wght' 520 survives only as long as someone keeps typing it. In CSS the bridge is font-weight, whose keywords and numbers map onto the weight axis directly, so font-weight: 600 usually lands exactly on the instance the family calls Semibold.

Two traps. First, an instance name is not a guarantee of position: one family’s Semibold is 600, another’s is 620, and a third calls 600 Demi. Read the font’s table, do not assume the ladder. Second, a named instance can pin more than one axis at once. A family with weight and optical size axes may name a point Caption Bold, meaning a specific weight and a specific optical size together, and picking it sets both. Asking for the weight alone leaves the other axis wherever the file’s default put it, which is how text set at display optical size ends up in a caption.