Backward compatibility

Backward compatibility is the ability to share data or commands with older versions of itself, or sometimes other older systems, particularly systems that a newer version intends to supplant.

Examples
Sometimes backward compatibility is limited to being able to read old data but does not necessarily extend to being able to write data in a format that can be read by old versions. For example, WordPerfect 6.0 can read WordPerfect 5.1 files, so it is considered to be backward compatible. It can be said that is backward compatible with, because Perl was (among other things) intended to replace AWK, and can, with a converter, run awk programs.

The Macintosh was the first example from Apple Computer that was not compatible with a preceding platform (the Apple II). To satisfy its large existing user base, Apple offered the Apple IIe Card to emulate an Apple IIe on supported Macs with LC slots. When Apple was transitioning classic Mac OS to Mac OS X, it introduced the Classic environment to allow users of newer systems to run older Mac software, though without access to more advanced features. During transitions from PowerPC to Intel and then Apple processors, Apple respectively introduced Rosetta and Rosetta 2 to provide backward compatibility with previous processor families.