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Legacy systems may be defined informally as “large software systems that we don't know how to cope with but that are vital to our organization”. Legacy software was written years ago using outdated techniques, yet it continues to do useful work. Migrating and updating this baggage from our past has technical and nontechnical challenges, ranging from justifying the expense in dealing with outside contractors to using program understanding and visualization techniques.
Keith Bennett (Sun,) studied this question.