A cord is (1) a string or rope, (2) an electrical cable, (3) a measure of wood equal to 128 cubic feet, (4) a ribbed fabric (short for corduroy) or pants made from the fabric, and (5) one of several types of cords found within the bodies of animals (e.g., the spinal cord and the umbilical cord). Chord is usually a musical term (though it is sometimes used metaphorically) denoting any combination of three or more pitches played at the same time, and it also has a few rare uses in geometry and science.
The cliché strike a chord is a metaphorical reference to music, so chord is the correct spelling.
A damaged extension cord caused a massive fire Sunday afternoon that destroyed Points West Sales and Leisure Sports. [PA Herald]
Rather he deploys it to float a perfectly weighted melody against a broken-chord accompaniment as unruffled and beautiful as a blue lagoon. [This is London]
It takes one cord of wood or 60 gallons of oil to boil 800 gallons of syrup. [Post-Gazette]
He calls the final chord of ” A Day in the Life,” from “Sgt. Pepper’s Lonely Hearts Club Band,” in a typical aside, “the greatest E major chord in the history of Western music.” [New York Times]