A fore-and-aft triangular mainsail achieves a better approximation of a wing form by extending the leech aft, beyond the line between the head and clew in an arc called the "roach", rather than having a triangular shape. This added area would flutter in the wind and not contribute to the efficient airfoil shape of the sail without the presence of battens. Offshore cruising mainsails sometimes have a "hollow leech" (the inverse of a roach) to obviate the need for battens and their ensuing likelihood of chafing the sail.