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Transition faults are used for modeling delay defects. A comparison between transition faults and single stuck-at faults indicates that many more transition faults than single stuck-at faults in standard-scan circuits are undetectable. Furthermore, this paper shows that undetectable transition faults in benchmark circuits appear in larger clusters than single stuck-at faults, where a cluster consists of several undetectable faults that are included in the same connected subcircuit. This implies that test sets for transition faults do not cover delay defects uniformly across the circuit. The paper studies the clustering of undetectable transition faults in standard-scan benchmark circuits by considering exhaustive as well as deterministic test sets. It defines double transition faults that provide targets for improving the coverage of subcircuits with undetectable transition faults, and presents the results of test generation.