The Three Classes

Pius IV (b. 1499, Pope 1559–1565).
Index librorum prohibitorum, cum regulis confectis per patres à Tridentina Synodo delectos, auctoritate sanctiss. D.N. Pii IIII, Pont. Max. comprobatus.
Lyon: Gulielmus Rovillius, 1564.
[Bound with:] Council of Trent (1545–1563).
Canones, et decreta sacrosancti oecumenici et generalis Concilii Tridentini.
Lyon: Gulielmus Rovillius, 1564. (AET7905)

The 1564 Tridentine Index defined three basic classes of prohibited books:
Class I: Books by heretical authors, all of whose writings are prohibited.
Class II: Individual prohibited books, classified alphabetically by author.
Class III: Anonymous prohibited books, classified alphabetically by title.

During its first century of existence the Index librorum prohibitorum listed condemned books in an alphabetical sequence subdivided into the three classes of prohibition. For example, in the exhibited Lyon edition of the 1564 Index, under the letter P, authors of Class I were listed first by name, books in Class II were listed next by author, and anonymous works in Class III were listed last by title. Beginning in 1664, every book in the Index was listed in a single continuous alphabetical sequence, regardless of its class.