Originally Posted by chornedsnorkack
Sure, it works for most main decks.
But Boeing 747 upper deck has the maximum width at floor. It then narrows rather steeply upwards. Nevertheless B747 has the upper deck.
Yes, but the seats have to be well inset from the fuselage walls, to give some headroom - at least one seat width is lost that way (at least, that was the case when I last travelled upper deck).