Multiple master fonts are customizable Type 1 fonts whose typeface characteristics are described in terms of variable design axes, such as weight, width, style, and optical size.

Some multiple master fonts include an optical size axis, which lets you use a font specifically designed for optimal readability at a particular size. Generally, the optical size for a smaller font, such as 10 point, is designed with heavier serifs and stems, wider characters, less contrast between thick and thin lines, taller x height, and looser spacing between letters than the optical size for a larger font, such as 72 point.