Font Selection

Before Emacs can draw a character on a graphical display, it must select a font for that character¹. See Fonts. Normally, Emacs automatically chooses a font based on the faces assigned to that character—specifically, the face attributes :family, :weight, :slant, and :width (see Face Attributes). The choice of font also depends on the character to be displayed; some fonts can only display a limited set of characters. If no available font exactly fits the requirements, Emacs looks for the closest matching font. The variables in this section control how Emacs makes this selection.

— User Option: face-font-family-alternatives

If a given family is specified but does not exist, this variable specifies alternative font families to try. Each element should have this form:
          (family alternate-families...)
If family is specified but not available, Emacs will try the other families given in alternate-families, one by one, until it finds a family that does exist.

— User Option: face-font-selection-order

If there is no font that exactly matches all desired face attributes (:width, :height, :weight, and :slant), this variable specifies the order in which these attributes should be considered when selecting the closest matching font. The value should be a list containing those four attribute symbols, in order of decreasing importance. The default is (:width :height :weight :slant).
Font selection first finds the best available matches for the first attribute in the list; then, among the fonts which are best in that way, it searches for the best matches in the second attribute, and so on.
The attributes :weight and :width have symbolic values in a range centered around normal. Matches that are more extreme (farther from normal) are somewhat preferred to matches that are less extreme (closer to normal); this is designed to ensure that non-normal faces contrast with normal ones, whenever possible.
One example of a case where this variable makes a difference is when the default font has no italic equivalent. With the default ordering, the italic face will use a non-italic font that is similar to the default one. But if you put :slant before :height, the italic face will use an italic font, even if its height is not quite right.

— User Option: face-font-registry-alternatives

This variable lets you specify alternative font registries to try, if a given registry is specified and doesn't exist. Each element should have this form:
          (registry alternate-registries...)
If registry is specified but not available, Emacs will try the other registries given in alternate-registries, one by one, until it finds a registry that does exist.

— User Option: scalable-fonts-allowed

This variable controls which scalable fonts to use. A value of nil, the default, means do not use scalable fonts. t means to use any scalable font that seems appropriate for the text.
Otherwise, the value must be a list of regular expressions. Then a scalable font is enabled for use if its name matches any regular expression in the list. For example,
          (setq scalable-fonts-allowed '("iso10646-1$"))
allows the use of scalable fonts with registry iso10646-1.

— Variable: face-font-rescale-alist

This variable specifies scaling for certain faces. Its value should be a list of elements of the form
          (fontname-regexp . scale-factor)
If fontname-regexp matches the font name that is about to be used, this says to choose a larger similar font according to the factor scale-factor. You would use this feature to normalize the font size if certain fonts are bigger or smaller than their nominal heights and widths would suggest.

39.12.9 Font Selection

Footnotes