Many times I have seen people claim (or assume) that one can copyright their arrangement of a traditional tune. However, in all the reading I have done about copyright, I have never seen that verified.

The US copyright is available online as a PDF. Recently I searched it, and the words "arrangement" and "chords" do not occur in the law. That's a good indication that one cannot copyright an arrangement.

That makes sense, because even if somebody noted down the many, many (and ever-changing) notes in the typical modern accompaniment, Congress would never okay the money to store, search and protect all that data.