the lateen sail did not directly come from the square sail. there was an intermediate sail type called the lug sail. it was one of the earliest fore-and-aft rigs. it was developed for use on the indian ocean. sailors knew that the wind does not always come from behind the vessel. they learned that the square sail could be made more efficient by turning it on the mast so it remained at a right angle to the wind. when used with a keel and some form of steering, it provided more options. instead of simply sailing downwind, the vessel could move in different directions relative to the direction of the wind. if the wind came from the beam (side of the ship), rotating the sail became less effective. but, if the luff (edge of the sail) was pointed into the wind and kept taut, the sail would provide forward movement. this could be made more effective if the yard (or spar) holding the sail is sloped downward into the wind. from this discovery came the lug, then the lateen.