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Legacy code refers to an application system source code type that is no longer supported. Legacy code can also refer to unsupported operating systems, hardware and formats. In most cases, legacy code is converted to a modern software language and platform. However, to retain familiar user functionality, legacy code is sometimes carried into new environments.
There is a common, false perception that legacy code is old. Although some software developers look at legacy code as a poorly written program, legacy code actually describes a code base that is no longer engineered but continually patched. Over time, an unlimited number of modifications may be made to a code base based on customer demand, causing what was originally well-written code to evolve into a complex monster.
A software developer will recognize legacy code when a feature cannot be added without breaking another logic. At this point, the developers may begin lobbying for a new system.