A caltrop (also known as caltrap, galtrop, cheval trap, galthrap, galtrap, calthrop or crow's foot) is an antipersonnel weapon made up of two or more sharp nails or spines arranged in such a manner that one of them always points upward from a stable base (for example, a tetrahedron). Caltrops were part of defenses that served to slow the advance of horses, war elephants, and human troops. They were particularly effective against the soft feet of camels. In more modern times, caltrops are still effective when used against wheeled vehicles with pneumatic tires.