The principle of font variation is close to that of the Multiple Master fonts: we define several "axes of variation" and take a few points in the space formed by these axes—in other words, several combinations of the values of these axes. Example: if we define the weight and the optical size as axes, and if the weights range from −1 to +1 and the optical sizes range from 6 to 72, we can take the combinations (0, 10) = weight 0 and optical size 10, (−1, 10), (+1, 10), (−1, 6), (+1, 72), etc. To each of these pairs of values, there corresponds a version of the font.
In the case of Multiple Master, we made the distinction between master designs and intermediate values. That means that a designer could focus on several specific designs and let the system perform the requisite interpolations on the positions of the points to yield all the other possible variants.
Here the situation is slightly more complex. The master designs still exist, but they can differ for each glyph. Therefore a glyph that never varies will be described only once. The glyphs that vary only slightly will have only a few master designs. Those with many details will have more master designs. It is a genuine miracle if the processor manages to cope with all these values and generate a reasonable rendering.
Almost all the tables in this section, which is devoted to font variation, have a name that ends in var. Here it is fvar ("font variation"). Later ...