A
caltrop is a device made up of four (or more) sharp nails or spines that are arranged in such a manner that one of them is always pointing upward (eg, a
tetrahedron). In the military context, it has been used to slow down the advance of
horses and wheeled vehicles. Caltrops come in a wide range of sizes, from small caltrops only a few centimeters across that were used by
ninjas to deter humans in soft-soled shoes to massive steel or concrete devices designed to wedge into
tank treads.
Modern police and security forces often use spike strips[?] against vehicles with inflated rubber tires, a device similar to a patch of caltrops in principle.