A hybrid disc is a disc, such as CD-ROM or Blu-ray, which contains multiple types of data which can be used differently on different devices. These include CD-ROM music albums containing video files viewable on a personal computer, or feature film Blu-rays containing interactive content when used with a PlayStation 3 game console. Multiple file systems A hybrid disc is an optical disc that has multiple file systems installed on it, typically ISO 9660 and HFS+ (or HFS on older discs). One reason for the hybrid format is the restrictions of ISO 9660 (filenames of only eight characters, and a maximum depths of eight directories, similar to the Microsoft FAT file system). Another key factor is that ISO 9660 does not support resource forks, which is critical to the classic Mac OS' software design (OS X or macOS removed much of the need for resource forks in application design). Companies that released products for both DOS (later Windows) and the classic Mac OS (later macOS) could release a CD containing software for both, natively readable on either system. Data files can even be shared by both partitions, while keeping the platform specific data separate. In a "true" (or "shared") hybrid HFS filesystem, files common to both the ISO 9660 and HFS partitions are stored only once, with the ISO 9660 partition pointing to file content in the HFS area (or vice versa). Blizzard Entertainment has released most of their computer games on hybrid CDs. By default, Mac OS 9 and macOS burn hybrid discs. An ISO 9660/HFS hybrid disc has an ISO 9660 primary volume descriptor, which makes it a valid ISO 9660 disc, and an Apple partition. It may also have an Apple partition map, although this is not necessary. The ISO 9660 portion of the disc can co-exist with an Apple partition because the header areas which define the contents of the disc are located in different places. The ISO 9660 primary volume descriptor begins 32,768 bytes (32 KB) into the disc. If present, an Apple partition map begins 512 bytes into the disc; if there is no partition map, the header for an Apple HFS partition (known as a Master Directory Block, or MDB) begins 1,024 bytes into the disc. Audio CD with added data tracks Hybrid-CD also refers to an audio CD which also includes a data track storing MP3 (or other digital audio compression format)