Among the non-ecclesiastical authors banned were Machiavelli, Guicciardini and Boccaccio.

It is noteworthy that the _Decameron_ was expurgated not chiefly for its indecency but for its satire of ecclesiastics.
Thus, a tale of the seduction of an abbess is rendered acceptable by changing the abbess into a countess; the story of how a priest led a woman astray by impersonating the angel Gabriel is merely changed by making the priest a layman masquerading as a fairy king.
The principles upon which the prohibition of books rested were set forth in ten rules.

The most interesting are the following: (1) Books printed before 1515 condemned by popes or council; (2) Versions of the Bible; (3) books of heretics; (4) obscene books; (5) works on witchcraft and necromancy.
In order to keep the Index up to date continual revision was necessary.
To insure this Pius V appointed a special Congregation of the Index, which has lasted until the present day.

From his time to ours more than forty Indices have been issued.

Those of the sixteenth century were concerned mainly with Protestant books, those of later centuries chiefly deal, for the purposes of internal discipline, with books written by Catholics.