fonts/cc-by-sa/knowledge/glossary/terms/variable_fonts/content.md

Variable fonts—or, more specifically, OpenType Font Variations—are a relatively new font format that allow one font file to contain multiple stylistic variations. Weight, width, style, optical size, etc. can then be manipulated by the end user (i.e., designer) and thus avoid the strict delineations of “traditional” fonts.

The first example has a light weight and body-like optical size; the second has a heavy weight and display-like optical size (note the higher contrast); the third also has a similarly heavy weight, but with a body-like optical size.

There are three main benefits offered by variable fonts:

1. To compress: The unified delivery of multiple instances—such as different weights, widths, optical sizes—in a single font file that is much smaller than the total file size of a collection of individual font files.
2. To express: The control given to users to select axes values directly, such as a custom weight that is between two named instances of weight. For example, between Regular (400) and Medium (500), a weight of 427 might be just the right one to express a design intent perfectly.
3. To finesse: The control given to programs to select axes values indirectly, based on context. For example, to automatically select an optical size instance using the current point size.

The variables within variable fonts are controlled by axes or instances. If the type designer, chooses, any variable in the type design can be assigned to a user-controllable axis.