The Clementine Collection comprises combined personal libraries formed over several centuries by the Albani family of Urbino and Rome. This noble Italian family included diplomats and cardinals as well as Giovanni Francesco Albani, reigning as Pope Clement XI from 1700-1721. Collection strengths include church history, philosophy, and major Roman and canon law works together with a significant collection on Jansenism including unique material pertaining to this movement, to the 18th century Gallican church, and to the aftermath of the bull "Unigenitus." Also present are books of Italian literature and drama denoting a library of cultured men who were important patrons of the arts. A number of these works are significant for their printing, provenance (presentation copies) or physical features, (original bindings). Books date from the 15th century to the early 19th. An estimated eleven percent are 16th century imprints, with the largest percentage from the 17th and a slightly smaller number from the 18th century. An estimated 33 percent of the collection is not simply hidden from scholars but entirely unknown, having no listing in OCLC, the database accessed by scholars searching for books via WorldCat. Subject areas with the highest proportion of such unique titles include Jansenist material and Roman and canon law. Beyond their absence in WorldCat, many of these works are also unknown in the catalogs of European libraries and thus immune from online discovery.