backwards compatible

In telecommunications and computing, a product or technology is backward compatible or downward compatible if it can work with input generated by an older product or technology such as a legacy system. If products designed for the new standard can receive, read, view or play older standards or formats, then the product is said to be backward-compatible; examples of such a standard include data formats and communication protocols. Modifications to a system that do not allow backward compatibility are sometimes called “breaking changes.” The reverse is forward compatibility, which implies that old devices allow (or are expected to allow) data formats generated by new (or future) devices, perhaps without supporting all new features. A standard supports forward compatibility if older product versions can receive, read, view, or play the new standard. For example, the introduction of FM stereo transmission allowed backward compatibility since new FM radio receivers could receive monaural signals generated by old transmitters. It also allowed forward compatibility, since old monaural FM radio receivers could still receive a signal from a new transmitter. In programming languages, backward compatibility refers to the ability of a compiler for version N of the language to accept programs or data that worked under version N – 1. By this definition, if previous versions (N – 1, N – 2, etc.) were also backward compatible, which is often the case, then, by induction, version N will also accept input that worked under any prior version after, and including, the latest one that was not backward compatible. However, in practice, features are often deprecated and support is dropped in a later release, which is yet thought of as backward compatible. In other contexts, a product or a technology is said to be backward compatible when it is able to fully take the place of an older product, by inter-operating with products that were designed for the older product. A data format is also said to be backward compatible with its predecessor if every message or file that is valid under the old format is also valid, and retains its meaning, under the new.