As for statutes, problems had developed with the citation of acts by the regnal year of the parliamentary session in which they received the royal assent. Perhaps borrowing from the bibliographic reference, some attorneys encountered problems because they would declare that a statute had been enacted in, for example “2 and 3 E. 6.” The courts held that an act cannot be passed in two years, and consequently the variance was fatal. On the other hand, one lawyer, perhaps aware of some of these cases, tried to cite an act as 4 Philip and Mary. But Mary had been queen regnant for a year before she married King Philip of Spain, who was made titular King of England. So the first year of Philip was the second year of Mary, and there could be no fourth year of Philip and Mary. The act should have been cited as 4 and 5 Philip and Mary. No less a judge than Lord Mansfield held the citation a fatal variance, although he was apologetic.
—Byron D. Cooper, Anglo-American Legal Citation: Historical Developments and Library Implications, 75 L. Libr. J.3 (1982)