It was originally designed in 1977 for use on floppy disks. FAT was soon adapted and used almost universally on hard disks throughout the DOS and Windows 9x eras for two decades. As disk drives improved, the capabilities of the file system have been extended accordingly. This has resulted in three major file system variants: FAT12, FAT16 and FAT32. The FAT standard has also been expanded in other ways while generally preserving backward compatibility with existing software.