When an operating system (or software that comes with its own text rendering) needs to show text, it has to determine if the characters are available in the selected font. If a character is missing it will eirther show the .notdef glyph, or it uses a fallback mechanism e.g. it will look for the character in other fonts. A last resort font ensures at least one font is able to show such missing character, those fonts are also known as fallback fonts.

Character to glyph index mappings

Typically last resort fonts contain a glyph for a specific character group, where such glyph represents the specific range of characters. This will ensure the number of glyphs remain low, while the font can contain mappings to all Unicode planes covering over one million code points. Within the OpenType specification there are basically two formats which can store many character to glyph index mappings. The common format which works great for regular fonts and a compact format which is optimized for storing ranges of characters mapped to single glyphs.

Windows doesn’t support last resort fonts that make use of the compact format. Apple’s Last Resort font uses the compact format. It is pre-installed on a Mac and contains all Unicode characters. The file size of the Last Resort font is 3KB, but if saved with the common format it will become 12.7MB.

By default FontCreator saves fonts with the common format, unless you check the “last resort font” box.

Blank Fonts

Some web sites use a blank font to detect whether a font is present on the system of the visitor. Other sites use such font to ensure no characters are shown from another font.