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A machine representation of permanent memory faults is introduced where, if M is a sequential machine representing some fault free system, a memory fault is represented by a function μ on the states of M, and the result of the fault by an appropriately determined machine Mμ. Given this representation, the investigation is primarily concerned with faults that are tolerated (masked) in the sense that resulting behavior relates in some specified way to original behavior. Several types of fault masking are thus considered and conditions for their existence investigated. The restriction to a special class of stable faults is also studied, this class being of interest since it includes all faults which correspond to "stuck-at" failures in the memory cells of a sequential switching network.