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Variable fonts aren't really a separate format — they use the standard .ttf or .otf container with additional OpenType tables (fvar, gvar, STAT) that define continuous axes of variation. A single variable font file can contain every weight from hairline to black, every width from compressed to expanded, and any other axis the designer defines — italic, optical size, serif style, whatever.
The .vf extension itself is primarily used in TeX/LaTeX systems for virtual fonts (a different concept entirely), but the term "variable font" has become synonymous with OpenType Font Variations, introduced in OpenType 1.8 (2016). Under the hood, a variable font stores a default set of glyph outlines plus delta values that describe how each point moves along each variation axis. The rendering engine interpolates between these deltas to generate any intermediate style.
Variable fonts have transformed web typography. Instead of loading four separate font files (regular, italic, bold, bold-italic), a single variable font covers the entire design space — often with a smaller total file size. Google Fonts, Adobe Fonts, and independent type foundries now offer variable versions of popular typefaces. The `font-variation-settings` CSS property gives designers continuous control over weight, width, and custom axes.