Backward-compatible

Backward compatibility is a property of a system, product, or technology that allows for interoperability with an older legacy system, or with input designed for such a system, especially in telecommunications and computing. Backward compatibility is sometimes abbreviated to BC, or called downward compatibility. Modifying a system in a way that does not allow backward compatibility is sometimes called "" backward compatibility. A complementary concept is forward compatibility, which is a design philosophy, usually based on open standards, that strives for methods that will continue to work with newer and future products. Design that is forward-compatible usually has a roadmap for compatibility with future standards and products.

The associated benefits of backward compatibility are the appeal to an existing user base through an inexpensive upgrade path as well as the network effect, which is particularly important, as it increases the value of goods and services proportionally to the size of the user base.

The associated costs of backward compatibility are a higher bill of materials if hardware is required to support the legacy systems; increased complexity of the product that may lead to longer time to market, technological hindrances, and slowing innovation; and increased expectations from users in terms of compatibility.

A simple example of both backward and forward compatibility is the introduction FM radio in stereo. FM radio was initially mono, with only one audio channel represented by one signal. With the introduction of two-channel stereo FM radio, a large number of listeners had only mono FM receivers. Forward compatibility for mono receivers with stereo signals was achieved through sending the sum of both left and right audio channels in one signal and the difference in another signal. That allows mono FM receivers to receive and decode the sum signal while ignoring the difference signal, which is necessary only for separating the audio channels. Stereo FM receivers can receive a mono signal and decode it without the need for a second signal, and they can separate a sum signal to left and right channels if both sum and difference signals are received. Without the requirement for backward compatibility, a simpler method could have been chosen.

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