This chapter shows the condition which the generalization holds true and theories about why it is true. There are three main experimental methodologies in which repetition spacing has been studied extensively: free recall, continuous paired-associate learning, and the short-term memory distractor method. For completeness, it should be added in passing that repetition of items within a series has quite different consequences for ordered recall in the memory-span situation than it does within the free-recall paradigm. Multitrace theory maintains that two occurrences of an item lead to partially distinct memory traces. There are two ways of conceiving this distinction between multiple memory traces of the same nominal event: Time tagging theory suggests that some representation of absolute or relative time is part of the memory representation stored when an item occurs; for example, our loaf of bread could be marked "Tuesday". Context tagging theory suggests that events become associated with other events that have been occurring contiguously in time.