Reading Geoff Pullum's post about Harold E. Palmer's 1935 Gilbert & Sullivan adaptation, I wondered about the spelling of "vocal chords" in the passage
I wish to call attention to the tactics and the strategy
Exemplified in all the work produced by Dr. Chatterji,
To ascertain what happens at the back of people's pharynxes
And analyse the vocal chords in artificial larynxes.
The "vocal cords" are so named from the resemblance of the vocal ligaments to strings or cords, so I wondered whether the "vocal chords" spelling might be an antique eggcorn. The answer turns out to be "yes", but with a twist: cord and chord participated in a rare reciprocal swap.
The OED explains that cord meaning "a string or small rope" is "a 16th cent. refashioning" of chord n.1 from Latin chorda, Greek χορδή. On the other hand, chord meaning "agreement of musical sounds" is "[o]riginally cord, aphetic form of accord n., q.v.; the 17th cent. spelling chord arose from confusion with chord n.1" (which of course is what we now mostly spell "cord").
This all seems to have been in play before the standardization of English spelling — but unlike many similar confusions, it apparently was never fully resolved. Some residues remain, like the "chord" (or "cord") of an arc, or the "chord" of an airplane's wing.
It's interesting that this does not seem to have become a serious irritant for peevers. After all, essentially everyone is Doing It Wrong, at least from an etymological point of view: cord should be "chord" and chord should be "cord".
Of course, there are occasional complaints:
Edmund Shaftesbury, The two sexes: Their functions, purposes, and places in nature, 1870: She tells the story of courtship in various bits of reminiscence, from which we learn that old men, middle-aged men and youths actually endeavored to vibrate the tender cord (not chord) that led the way to her heart.
Flying, 1967: And in the specification box, you must have meant chord: not cord, unless he was trailing a piece of string.
Roy Copperud, American Usage (1970): The folds in the throat that produce the sound of the voice are vocal cords, not chords; also spinal cord (not chord).
But on the whole, this historic confusion seems to be tolerated without much fuss.
There's an excellent discussion (as usual) in Merriam-Webster's Dictionary of English Usage, under chord, cord (p. 242-3), which concludes that
… even though the chord spelling with adjectives like vocal and spinal is historically justified and considered acceptable by a number of British authorities, it is widely understood to be a misspelling in American usage. While it is not really a misspelling, we recommend that you use the commoner cord spelling.