A Variable Fonts is made of two or more master files, with compatible contours of all glyphs across all masters. For each glyph, compatibility consists of:

• having the same number of contours and points;
• contours that have the same direction with the exact position of the starting point;
• and are set in the same order.

That way, there can be a fluid transformation (called “interpolations”) between every master of the Variable Font.