Given the limitations of sailing large, square-rigged ships of war, the line of battle – columns of heavily armed ships in parallel opposite each other at close range – was the preferred way to bring as many guns to bear at once against an opponent during fleet actions at sea. The movements of these ships were constrained by the wind, because square-rigged ships had limited capabilities in upwind sailing and maneuvering. The masts and rigging of these ships also interfered with the radius of fire by their guns. It was difficult to place guns on the upper deck where they could rotate and fire freely in any direction. Instead, the main gun batteries were arrayed in rows on either side of the ship, where they fired through open gunports. This arrangement, which evidence from shipwrecks like the Mary Rose shows was in use by at least the 16th century, meant that individual guns were restricted in their ability to elevate and traverse. The positioning of one's ship relative to one's opponent was more important in bringing the guns to bear than the aim or accuracy of the gun. Another limitation imposed by this mode of warfare at sea was that the black powder used produced prodigious quantities of smoke that enveloped fighting ships after their first broadside. This often made it difficult for opposing ships to see each other well enough to maneuver and train their guns.