Q. My publisher prefers that AD come before the year (as in “AD 99”), but would the same rule apply to centuries? That is, should it be “first century AD” or “AD first century”?
A. The abbreviation AD stands for anno Domini, Latin for “in the year of the Lord,” so the order AD 99 is required for proper syntax: “in the year 99” makes sense; “99 in the year” does not. BC, which means “before Christ” (the English-language complement to AD), naturally follows the year: 99 [years] BC. The problem with AD and centuries is that they don’t mix: “in the year of the Lord first century” and “first century in the year of the Lord” both fail the test. Not to worry. English and Latin were never meant to coexist, and “the first century AD” is a perfectly acceptable way of referring to “the first century after the year 1 BC” (there is no year zero). (The alternative, “the AD first century,” doesn’t quite work.) Some writers use CE ([of the] Common Era) and BCE (before the Common Era), both of which follow the year, but the older abbreviations have persisted and are more likely to be understood by readers. See CMOS 9.34 for some additional considerations.