The SYSTEM subdirectory of a user's Windows directory (the directory in which Windows is installed) contains the system's font resource files. A font resource file is an empty Windows library; it contains no code or data but does contain resources.
Raster and vector font resource files are identified by the .FON filename extension. TrueType font resource files have the .FOT filename extension. Each .FOT file is a relatively short header that refers to a file containing TrueType font information. These TrueType font-information files have the same base filename as the .FOT files, but have the .TTF filename extension.
Some of the filenames for raster and vector fonts are followed by a lowercase letter that indicates the resolution for which the font was designed. This letter varies according to the type of display device that was specified when the fonts were installed. Following are the lowercase letters used to identify different resolutions:
| Letter | Device |
| a | CGA |
| b | EGA |
| c | Okidata printers |
| d | IBM and Epson printers |
| e | VGA |
| f | IBM 8514/A |
For information about defining and using a specialized font resource, see Section 18.4.9, “Creating Customized Fonts.” For more information about the format of font resource files, see the Microsoft Windows Programmer's Reference, Volume 4.