I don't know at what level this is used, but I have only ever seen it in my graduate level logic and mathematics courses. (I'm European.)<p>I like it. Part of a good proof is a rigorous derivation. There is plenty of thing sto be said about what intuition can lead us towards such a proof. But at the end of the day, a good proof is ideally almost machine-checkable. Most proofs (much like code) lives two lives: the one in which they are written, and the one in which they are read. It should be almost trivial for a student to go back, on their own, days or weeks later, and see that each step is valid. And I think prose interspersed between column-style proofs do a better job than what margin notes do. The reason two columns work well is that the step and it's justification are of both condensed and almost always of similar length. Justification (much like code comments) may or may not need a good chunk of space. And surely nothing is stopping anyone from inserting little margin notes here and there in a two-colum proof, right?