Systems that do not offer a fallback font typically display black or white rectangles, question marks, or nothing at all in place of missing characters. Symbols in a fallback font can contain annotations such as the relevant Unicode block and the script system used.
LastResort is a Mac OS font that is invisible to the end user, but is used by the system to display glyphs that are not available in any other font. The symbols provided by LastResort place glyphs into categories based on their location in the Unicode system and provide a hint to the user about which font or script is required to view the unavailable characters. The symbols provided by LastResort are square with rounded corners with a bold outline. In the left and right sides of the outline, the Unicode range that the character belongs to is given using hexadecimal digits. Top and bottom are used for one or two descriptions of the Unicode block. A symbol representative of the block is centered inside the square.
Apple's LastResort font was first included in Mac OS 8.5 in 1998, for the benefit of applications using Apple Type Services for Unicode Imaging (ATSUI). It is also used in Mac OS X. In 2001, for the second release of OS X, the Last Resort font design was revised to include the border text and was re-digitized, and extended by Michael Everson of Evertype, who continues to update it with each new release of Unicode. Apple has now made the Last Resort font available for free download from the Unicode website.
One prototypic glyph per Unicode block is used because the total number of Unicode characters greatly exceeds the address space of an sfnt (TrueType and OpenType) font structure, which has a 16-bit glyph index that can store a maximum of only 65,536 glyphs. Unicode has now over 100,000 defined characters, with a potential address space of over one million characters - over 15 times the sfnt size limit. Using this one-glyph-per-block generalization, the Last Resort font is still capable of showing a glyph for every byte in Unicode. The Last Resort mechanism will therefore not break as Unicode continues to grow and the Basic Multilingual Plane (BMP) and surrogate planes fill up further.
The font was created for debugging purposes.