The first sails were probably square rigs – that is, square sails hung from a beam or yard from the mast, across the boat at right angles. These were simple and effective. It’s easy to appreciate how the wind drives the boat along when the boat is travelling “downwind” – that is with the wind behind. The wind simply pushes against the sail and the boat “runs” before it. Because the wind is behind the boat, the boat remains stable, despite the area of sail and the mast on top.