The spelling checker uses two types of dictionaries, predefined and custom. Predefined dictionaries have the extension .adm, e.g. American.adm, which is supplied with the checker. They are comprehensive, highly-compressed dictionaries for their own language or dialect (e.g. American English in this example) but cannot be modified. Custom dictionaries include the spelling checker's own built-in Ignore All ⁄ Change All dictionary and a user Custom dictionary (created at installation), bearing the name of the program's owner (in this example, Fred) with the extension .adu. You are free to customize these dictionaries, hence the name.