Bush regarded the notion of “associative indexing” as his key conceptual contribution. As he explained, this was “a provision whereby any item may be caused at will to select immediately and automatically another. This is the essential feature of the memex. The process of tying two items together is the important thing.” This “linking” (as we now say) constituted a “trail” of documents that could be named, coded, and found again. Moreover, after the original two items were coupled, “numerous items” could be “joined together to form a trail”; they could be “reviewed in turn, rapidly or slowly, by deflecting a lever like that used for turning the pages of a book. It is exactly as though the physical items had been gathered together from widely separated sources and bound together to form a new book”