Labbé's _Bibliotheca bibliothecarum_ is an alphabetical list according to first names of some eight hundred authors of bibliographies. According to Besterman, it includes about fifteen hundred titles. Labbé's arrangement according to first names causes no difficulty to a modern user because he provides an index of family names. Unfortunately, however, this index of family names is incomplete, and the lack of care in its preparation is evidence that Labbé hurried to get his book to the printer. Eight subject indexes--only the fourth is not alphabetical--make the _Bibliotheca bibliothecarum_ fully usable. When the reader has familiarized himself with them (and apparently very few have done so), he can understand the book and its value to a scholar. In working with the indexes he will discover that Labbé did not make them complete and reliable. Part of the difficulty in understanding and using the indexes arises from Labbé's old and unfamiliar classification according to men instead of subjects. This classification was firmly established when Labbé wrote and he probably never thought of any other. St. Jerome had called his book by the alternative title "De scriptoribus ecclesiasticis," that is to say, "Ecclesiastical Writers," but it was a bibliography of ecclesiastical literature. A bibliography of astrology was, in Labbé's conception, a list of men who wrote on astrology. He soon ran into difficulties and adopted devices to get around them that show bibliographical method in a transitional state. In "Index I. Practitioners of Various Arts and Sciences (Index Primus. Professores variarum scientiarum atque artium representans)" we find such entries as "Advocatorum Consistorialium, Advocatorum Parisiensis Curiae, Aristotelis Graecorum Interpretum, Arithmeticorum," which we can translate (changing to the nominative case) as "Consistorial Lawyers, Lawyers of the Parisian Court, Greek Interpreters of Aristotle, Arithmeticians." These designations are to be understood as references to as many subjects. "Index II. [Bibliographies of] Nations and Countries" and "Index III. [Bibliographies of] Religions and Religious Orders," which does not include non-Christian religions or heretical sects, give him no trouble. In the fourth index Labbé meets his Waterloo. This "Index IV. Authors Writing on Various Subjects" is awkwardly conceived in terms of the authors but is arranged according to the theological merit of the subjects on which they wrote. It descends from the Virgin Mary to inventions in the following order: (1) writers about the Virgin Mary, (2) [writers about] the Immaculate Conception, (3) writers who were popes, (4) writers who were cardinals, (5) writers who were French cardinals, (6) women writers, (7) writers about heretics, (8) writers on the prohibition of heretical books, (9) compilers of catalogues of manuscripts, (10) compilers of catalogues of ancient and modern libraries and writers on library science, (11) writers on academies,