Hierarchical File System (HFS) is a proprietary file system developed by Apple Inc. for use in computer systems running Mac OS. Originally designed for use on floppy and hard disks, it can also be found on read-only media such as CD-ROMs. HFS is also referred to as Mac OS Standard (or, erroneously, "HFS Standard"), while its successor, HFS Plus, is also called Mac OS Extended (or, erroneously, "HFS Extended"). With the introduction of OS X 10.6, Apple dropped support to format or write HFS disks and images, which are only supported as read-only volumes.