We often hear about people who are starting or adding to rare book collections, but it can be difficult to know exactly what counts as a “rare” book. Indeed, you may find yourself wondering: What is a rare book? The Oxford English Dictionary (OED) defines something that is “rare” in a variety of ways, and some of them certainly apply to rare books. For example, the OED defines a rare object as “a thing or things . . . occurring infrequently, encountered only occasionally or at intervals, uncommon,” or “of a kind seldom found, done or occurring; unusual, uncommon, exceptional.” These definitions can help us to understand why a book might be rare—it is found infrequently, is uncommon to see or to hold, and is exceptional in some capacity. Yet, as you might guess, not all rare books are exceptional in the same ways.