The column.

The person in the middle, and both of them. The aisle passenger can lean into the aisle and the window passenger can lean into the wall. The middle passenger has nowhere to go, so the two armrests are theirs. I never had a passenger argue the point once it was explained, and most of them looked relieved that somebody had a rule.

The armrest is also a boundary. Whatever seat you are in, nothing of yours crosses it, and that includes your knees. If you are in the aisle or window seat and would like a turn on the armrest, ask. The middle-seat passenger will usually share, because they were asked. That is the whole difference.

You may ask, but you may not expect a yes. The seat reclines because the airline built it that way, and the person in front of you paid for that seat too. Ask before takeoff, and give a reason with the request: "I'd be grateful; I have a laptop out for the next hour." If the answer is no, that is the end of it. Thankfully, many people find it a pleasure to help another traveler.