FLEX a user interface to relational databases, can be used satisfactorily by users with different levels of expertise. FLEX is based on a formal query language, but is tolerant of incorrect input. It never rejects queries; instead, it adapts flexibility and transparently to their level of correctness and well-formedness, providing interpretations of corresponding accuracy and specificity. The most prominent design feature of FLEX is the smooth concatenation of several independent mechanisms, each capable of handling input of decreasing level of correctness and well-formedness. Each input is cascaded through this series of mechanisms until an interpretation is found. FLEX is also cooperative. It never delivers empty answers without explanation or assistance. By following up each failed query with a set of more general queries, FLEX determines whether an empty answer is genuine, in which case it suggests related queries that have nonempty answers, or whether it reflects erroneous presuppositions on the part of the user, in which case it then explains them. >