ANSWER: Subpoenas duces tecum. This phrase—like any other containing a postpositive adjective—takes its plural on the noun at its beginning, the phrase’s “head.” Similar plurals include these:
acts malum in se
brothers-in-law (but my in-laws)
corporations de facto
corporations de jure
mayors pro tem
Of course, some of these are antiquated: today, we tend to refer to defendants rather than parties defendant.
Meanwhile, over decades and centuries, the headword can shift. Even though we say sums certain, most literate writers would probably use sum totals, just as they would use totals as a plural noun when needed.
Fortunately, we have a limited number of postpositive adjectives in English. No matter how many sticklers for grammar might inhabit a single law office, these pluralizing points seem unlikely to foment battles royal.
By the way, a story in The Onion (9-20-2000) placed the late language maven William Safire in a Manhattan Burger King ordering “two Whoppers Junior.” In an online discussion transcribed and published in The Washington Post (4-23-2008), Safire acknowledged ordering “Whoppers Junior” — but only “because it gets a funny look or a laugh.”
Garner’s Dictionary of Legal Usage 685, 692, 856 (3d ed. 2011).
Garner’s Modern American Usage 638, 648 (3d ed. 2009).
Thanks to Cecelia Soboleski and Michael Starkman for suggesting this topic.
Bryan A. Garner, President of LawProse Inc., is the most prolific CLE presenter in the U.S., having trained more than 150,000 lawyers and judges. His book — most prominently Black’s Law Dictionary and Garner’s Modern American Usage — have been cited as authority by every state and federal appellate court, including the highest. For more about him, go to www.lawprose.org. To follow him on Twitter: @bryanagarner.