A dagger is essentially a special form of knife, where the tang is placed along the center line of the blade. A dagger is much more than an ordinary knife a weapon made for thrusting rather than a tool for slicing and cutting. Historically daggers were important secondary weapons in Europe during the Middle Ages and the renaissance.
Often a dagger is fairly long, and some may verge on being of sword length. Most daggers are double edged, although there are exceptions.