The medieval bastard sword made it's appearance at a transitional time in armor evolution and innovation. During the later part of the 14th century armorers started creating full articulated iron suits to protect their knights. Unlike the previous armor's this plate armor protected the knight from the blunt force injuries that often occurred when a single hand sword struck maille. Not only would it protect them from the force of the blow but it made cuts ineffective giving the iron knight an insurmountable advantage on the battlefield.
As it goes for arms and armor there is a delicate dance between offense and defense and when one is improved the other must answer. If you wanted to survive this new breed of warrior new measures needed to be created quickly. The answer was the bastard sword, also referred to as a hand-and-a-half sword. This sword was nick named the bastard because it had no family. It could not be classified a single hand sword and it it was not a two handed great sword. The bastard sword has a long enough handle that it could be used two handed but is light enough and balanced enough that it could be used with a shield. This would enable more men to carry them without sacrificing their defense.
The era is often depicted with battlefields full of armored knights on horseback but the reality was a battlefield which consisted of several moderately armored soldiers, pheasants and farmers forced to fight followed by a small number of armored knights. If you carried a bastard sword, wore some maille and carried a shield you would be prepared for almost anyone you may encounter. The bastard sword was made to be long and light for maneuverability and is effective for slashing but was built to thrust. If you encountered a knight slashing is ineffective because he can't be cut like other opponents. You would thrust or dig your tip into the articulated pieces trying to penetrate the suit. You would thrust into less protected areas, like the armpits, the inside of the elbows, the neck and even the openings in the helm. A long blade narrowing toward the tip with a distil taper meaning it becomes thinner toward the tip would give you the advantage needed.
Even against less armored opponents the bastard sword was a great innovation. It's length would give the wielder a reach advantage over a single handed sword, a mace or a hammer. The controllable well balanced tip would give an accuracy advantage digging into plate or maneuvering around a shield. It's blade is fast, due to the narrowing and thinning that it would give a speed advantage over many other weapons. The tip was thin enough and strong enough that it didn't just answer the new plate emerging but was quite effective thrusting into mail. The bastard sword was a great innovation and a key armament during the 100 years war between England and France that took place in the mid 14th- 15th Century.