Why do so many federal statutes use the spelling subpena instead of subpoena?
Funny thing. It seems to be the result of an old choice made for the Government Printing Office Style Manual. The earliest copy we have at LawProse, the 1926 edition, has no reference to the spelling of this word. But the 1973 edition recommends subpena and subpenaed. The change seems to have come sometime in the 1940s. The heyday for this spelling in all sorts of print sources was 1945 to 1955. Since then, it has dwindled. And in 1984 the GPO Style Manual (as it is abbreviated) changed its preference to the standard spelling subpoena.
So it all depends on when a given statute was enacted.
The midcentury idea, apparently, was to eliminate Latinate ligatures known as “digraphs.” (Some people mistakenly call them “diphthongs”: they’re actually digraphs—two vowels traditionally joined into a single character.) The tendency in American English has been to use spellings such as anesthetic (not anaesthetic), encyclopedia (not encyclopaedia), orthopedic (not orthopaedic), and penitentiary (not poenitentiary), etc.
Some words, though, are resistant. Aesthetic and subpoena are good examples. In 1943, as part of the anti-digraph wave, the hefty Funk & Wagnalls New Standard Unabridged Dictionary of the English Language, in two volumes, listed its main entries under esthetic and subpena. These were poor predictions on the part of some very good lexicographers. Neither American English nor British English has adopted the reformed spellings.
But for a time, the GPO Style Manual adopted them both, so we’re left with some anomalous spellings in our statute books. That all changed in 1984. Shades of George Orwell.
Garner’s Dictionary of Legal Usage 856 (3d ed. 2011).