What is the purpose of the blade fuller? The fuller is a groove forged or cut into the steel of a blade. Contrary to the silly but persistent myth it is not a “blood groove” and has nothing whatsoever to do with letting the blood flow or breaking the suction from a wound. The fuller accomplishes several things: it lightens the blade; it increases the stiffness of wider blades; and it allows a blade to have thicker edges for toughness.
Lightening the blade is rather obvious. Less steel, less weight. The number and width of the grooves varied considerably. In general early period pieces had a single wide fuller on each side of the blade and later period pieces had more narrow fullers. Fullers weren’t always positioned directly opposite each other on either side of the blade although on blades prior to about 1550 A.D. this was overwhelmingly true. Viking swords in particular are typical in having a single wide fuller on opposite sides of the blade.
If you have a wide blade, that is one that is more than about 2 inches wide, depending upon the thickness of the blade it may lose a bit of stiffness due to the necessarily long and thin bevels. One way to solve this is to have a fuller. By adding a fuller on a wide blade you can in effect create two spines and increase the stiffness of the blade by having shorter, thicker bevels.
One last bonus of having a fuller is that by placing the spine closer to the edge you make the bevel thicker and this increases the strength of the edge by having more material present. Can you have too much material near the edge? Definitely! Too much steel creates too thick of a bevel and seriously (no pun intended) cuts down the blade’s ability to move through a target.