Also known as a hand-and-a-half sword. A bastard sword has a blade length of ~30"-44" and a grip (the part of the hilt that you grasp) that is long enough to accomidate two hands, but the weight (~2.75-4.25 pounds) is such that the sword can be used with either one or two hands. Versatility is a Good Thing. Longswords are only meant to be wielded with one hand, and two-handed swords are only meant to be wielded with two hands.

Named the Bastard Sword because only noblemen could carry a true Great Sword. The illigitimate sons of noblemen carried swords the same size as their fathers, dubbed "Bastard" swords for the bastard sons who carried them.

Light enough you can move it quickly (ymmv), heavy enough your opponent will have a hard time parrying it with a saber or other relatively light blade. It can be wielded one-handed if you wish to keep your other hand free for balance (important!), shield wielding, or anything else you need to do. It can be wielded with two hands for additional speed (especially important against a lighter weapon), armor-penetration (if necessary at all), or firmness of parry - one can absorb the shock of a parry much more easily with both hands on the hilt.

Wielding a bastard sword, it feels massive yet elegant. Everyone should try it some time. Well, maybe not everyone.

A type of sword first introduced in the early 1400's. C.E., It's the hardest type to define with the designation usually meant in terms of a set of qualities that were being introduced to the field in that era, hence the name.

As a generalization, they usually had the same length as long-swords, 40-48 inches, and nearly the same weight, 3-3.5 pounds; but they differed in the switch-hitting handle that could be comfortably used in a one-handed or dual-handed fashion, and the inclusion of annelets and side-rings. These improvements were added to further the new need for better point control and maneuverability to catch up to the advancements in armor and foil them.

Resources:

Medieval Swordsmanship, by John Clements

European Weapons and Armor, by R.E. Oakeshott