Sometimes people come to me and ask, "How did medieval filmmakers protect their DVDs from piracy?" And I tell them that since so few households had DVD players during the thousand or so years between the fall of the Roman Empire and the Renaissance that it really never became much of an issue.

But this is not to say that the medievals didn't face problems safeguarding their intellectual property. Indeed, book owners were so worried about theft and damage to their property that they often i...