A standard CD-ROM file system. Evolving from the earlier High Sierra specification, ISO 9660 specifies the directory format of the disc, while the physical format is defined in the Yellow Book. ISO 9660 provides for read-only interoperability between Windows, Mac and Unix platforms. ISO 9660 Levels Level-1 guarantees the most interoperability, but requires file names to be upper case and limited to eight characters with a three-character extension similar to DOS file names, but even more restrictive (only alpha, numeric and the underscore character may be used). Level-2 and -3 support long file names with more special characters, but may not be supported by every drive. See ISO, ISO image, Joliet file system, Rock Ridge file system and UDF.