Q. I have two questions about the use of AD (anno Domini). First, is it acceptable to leave the abbreviation after the year when it refers to a decade, as in “the 30s AD” (referring to the fourth decade)? Or should that be “the AD 30s”? Second, since AD literally means “in the year of the Lord,” should we avoid saying “in AD 60,” etc., just as we avoid saying “in ibid.”?

A. When a span of years is expressed in the form of a decade, a century, or a millennium, it can safely precede rather than follow “AD.” So write “AD 30” but “the 30s AD” (or “the thirties AD”; see CMOS 9.33), “the second century AD,” “the first millennium AD,” and so forth.

Putting “AD” before the year in “AD 30” and “AD 30–35” and the like (as described in CMOS 9.34) does try to accommodate the Latin phrase behind the abbreviation (one would write “in the year 30,” not “30 in the year”). But Latin antecedents can take you only so far in English. It’s perfectly fine to write “in AD 2020” (despite any apparent redundancy). Likewise, though “ibid.” means “in the same place,” there’s nothing wrong with writing, for example, “referred to in ibid.” (but see CMOS 14.34, which discusses alternatives to “ibid.”).