A caltrop is an ancient antipersonnel weapon made up of four sharp nails or spikes arranged in such a manner that one of them always points upward from a stable base (for example, a tetrahedron). Used in large numbers, it was the defensive "landmine" of antiquity.
Caltrops served to slow down 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 used against wheeled vehicles with pneumatic tires.
But a caltrop is also an interesting subject for wood turning. The challenge is to complete the project without displaying any turning artefacts like holes or spigots. This one, in walnut, still has the mounting holes visible
There are four turning axes - corresponding to the points of a tetrahedron, so a little trigonometry is needed to get the angles just right