Fuller (groove)
A fuller is a rounded or beveled longitudinal groove or slot along the flat side of a blade (e.g., a sword, knife, or bayonet) that is made using a blacksmithing tool called a spring swage or, like the groove, a fuller. A fuller is often used to widen a blade. When combined with proper distal tapers, heat treatment and blade tempering, a fullered blade can be 20% to 35% lighter than a non-fullered blade with minimal sacrifice of strength or blade integrity. This effect lessens as the blade is reduced in length. A blade is said to  be "fullered" after introduction of the groove. Fullers are historically known as blood grooves or blood gutters, although channeling blood is not their purpose. Etymology. The term "fuller" is from the Old English "fuliere", meaning 'one that fulls [pleats] cloth'. It is derived from the Latin word "fullo". The first recorded use of the term in relation to metal working is 1587. The first recorded use of the term to describe a groove or channel in a blade is 1967. Tool. As a blacksmithing tool, a fuller is a type of swage. A swage is a tool that has a  particular shape forged into its surface, which is used to imprint that shape into the metal. Swages are often tools with handles and various shaped heads, which are placed between the hammer and the workpiece, allowing the smith to work the steel into various shapes that would be impossible to make with a hammer alone. Swage blocks are anvil-like blocks with various shapes forged in them, also for forming metal. The term "swage" is from the Old French "souage", meaning 'decorative groove' or 'ornamental moulding'. Fullers, in particular, are swages that have a cylindrical or beveled face, typically three  to six inches long, which are used for imprinting grooves into the steel. If a groove is to be applied to both sides of the steel, two fullers may be used at the same time, sandwiching the workpiece in the middle. Often, one fuller will have a peg that holds it securely in the anvil, while the other fuller will have a handle and a flat head, for striking with a hammer. Sometimes the two fullers may be connected with a flatbar of spring steel which is bent into a wide, "C" shaped bracket, keeping the two fullers aligned with  each other and freeing the smith's hands for manipulating the workpiece and hammer. A blade being fullered will generally be slowly pulled through the fullers as they are being hammered, until the entire groove is the proper depth and shape. As the fullers make the groove, they typically displace material to the side, creating ridges around the grooves. These ridges may be hammered flat, widening the blade, or they are often shaped with other swages, increasing the strength of the blade by creating thicker areas in its cross section. Fullers have a variety of uses beyond bladesmithing. They are  often used for "drawing out" steel, which consists of hammering a short block into a long bar. The fullers will often be used to notch the block or bar. If the fullers are longer than the block is wide, they will tend to lengthen the block rather than raise bulges from the surface. After notching, the block is hammered smooth, elongating it further, and this process is repeated until the bar assumes the correct length and thickness. Fullers are also used for other shaping processes in blacksmithing, from making hinges and latches to plow parts and horseshoes. Japanese blades. In  Japanese swordsmithing, fullers have a rich tradition and terminology, enough that there are separate terminologies for the top ("hi", usually pronounced as "bi" when used as the second member of a compound) and bottom ("tome") ends of the feature. The kukri. The Nepali kukri has a terminology of its own, including the "aunlo bal" (finger of strength/force/energy), a relatively deep and narrow fuller near the spine of the blade, which runs (at most) between the handle and the corner of the blade, and the "chirra", which may refer either to shallow fullers in the belly of the blade  or a hollow grind of the edge, and of which two or three may be used on each side of the blade.