One comes across this term when writing a CD. ISO 9660 is a file format for CDs. It was necessary to create a standard file system since competing operating systems created CDs that were not compatible with other operating systems. The ISO 9660 file format is an old format and is used in CDs since it guarantees maximum compatibility with standalone CD players. This standard format has been extended to increase its functionality. The Joliet extension allows the use of filenames larger than eight letters and containing non-ASCII characters. The El Torito extension allowed optical media to be made bootable. ISO 9660 is being replaced by UDF.