Incorporating techniques from book history into traditional intellectual history, this article traces the effective origin of indexing to the early printed editions of two lexicographical works, Lorenzo Valla's Elegantie and Niccolò Perotti's Cornu copiae, and then follows its development through the editions of the Roman poet Virgil published between 1500 and 1800. Indexing practices turn out to be tied to how books were read, with a new way of consuming books, which is labeled "transverse reading," emerging during this period.