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The properties of combinational logic functions and networks that influence equivalence among stuck-type faults are investigated. It is shown that the equivalence of certain types of faults depends only on the function being realized. For instance, the fault classes among primary input/output faults are of this type. It is shown that every irredundant realization of the two-variable EXCLUSIVE-OR function has a unique set of ten fault classes. A fault class F in a module M contained in a network N is called intrinsic, if F can be determined from M alone, i. e., F is independent of N. Using the concepts of intrinsic equivalence and inversion parity, conditions for the equivalence and nonequivalence of two fault classes are obtained. These results are applied to the problem of equivalence identification in two-level logic networks where they provide a substantial reduction in the amount of computation required.