As arms and weapons evolved knights could find themselves in situations where they might enter a battle on horseback but leave it on foot, thus the need arose for a sword that was short enough to wield on foot and yet long enough to reach targets from horseback. Enter the longsword, which was really just a longer and heavier version of the typical sword. These blades were effective against plate armored foes as well as being devastating against lightly armored soldiers. Used with two hands, they generated power; however, knights sometimes preferred to use them with one hand and kept a shield in the other. The biggest longswords were known as great swords whose sheer size made them ineffective on horseback. However, greatswords saw infantry action from the 13th century up to the early Renaissance and are viewed as the predecessors to the two-handed swords.
Contrary to popular belief, two-handed swords are not medieval weapons and differ from both longswords and great swords. Technically, the two-handed sword belongs to the Renaissance period. It was popular during the 16th century with Swiss and German infantrymen. These swords could be over six feet long and even at a relatively light 4 –6 lbs, you had to be a preternaturally strong human specimen to brandish it effectively with one hand. The German Zweihander was one such sword. The English Slaughter-sword was another. These weapons had surprisingly good balance and were destructive with wide sweeping blows. Primarily used to counter long weapons such as halberds and pikes, their great length meant that two-handed swords could also take the role of spears. Of course, only the strongest wielded them and these men were duly compensated (sometimes paid twice the regular soldier’s salary) for their troubles.
Check out more on historical and fantasy swords.