Variable Fonts technology allows users to navigate between two or more specific styles (called masters or sources) with high precision and much smaller font file sizes than several static fonts.

Outline compatibility allows the interpolation of vector shapes, enabling the generation of intermediate instances between two source files. This is essential for Variable Fonts, but even in static font design, maintaining compatibility is beneficial: it spares the designer from manually draw in-between weights or styles as separate masters, when they can be automatically created during the export.

For each glyph, compatibility means:
• same number of points, in the same order;
• same number of contours, in the same order and direction;
• same number of components, in the same order.