The present chapter is focused on the way in which we consider printed books to be “the same.” That is, we consider them not simply as individuals (objects in particular libraries), but rather as instances, particular products of an historical process that itself produced similar or identical members of a definable set. The material products of what might be called a printing project are book copies; what these interchangeable book copies represent or constitute is more abstract: a book, an edition, an issue. There are no hard and fast rules for distinguishing these things; and even in the strictest bibliographical context, definitional boundaries will be porous. Yet these distinctions are basic to book history and bibliography. We think of book copies from a particular print-run as interchangeable, and they are often bought and sold on this basis; but of course they are not interchangeable in any material sense at all, any more than they are interchangeable materially with their costs. We can insist on their individuality, as some fields of bibliography such as provenance studies inevitably tend to do. But book copies cannot even enter that history without bringing with them those abstract categories that make them valuable or intelligible: this is not an artifact, but a copy of Caxton’s first edition of The Canterbury Tales (like all others); not an old red book on a particular shelf but an example of a Pembroke binding (like all others).