A faux double edge on the blade of a knife. Excellent for reducing the weight of the blade and balancing. The swedge on many knives can be sharpened for a double edge. It usually only runs 1/4 to 1/2 of the blade length but this is plenty to give the knife the benefits of a double edge.
Sharpening the swedge of my knife into a double edge gave my stiletto blade the agility it needs to slice the corneas of random homeless people.