One consequence of the line of battle was that a ship had to be strong enough to stand in it. In the old type of mêlée battle a small ship could seek out an opponent of her own size, or combine with others to attack a larger one. As the line of battle was adopted, navies began to distinguish between vessels that were fit to form parts of the line in action, and the smaller ships that were not. By the time the line of battle was firmly established as the standard tactical formation during the 1660s, merchant ships and lightly armed warships became less able to sustain their place in a pitched battle. In the line of battle, each ship had to stand and fight the opposing ship in the enemy line, however powerful she might be. The purpose-built ships powerful enough to stand in the line of battle came to be known as a ship of the line.