The cone can be thought of as a double pyramid (with the base of one pyramid atop the other). In this case, the cone is constructed by taking a square base and adding to it four right triangles that lie on each side of the square.
The sides of these triangles are perpendicular to the sides of the square, so they form right angles with each other. The height at which they meet is called the slant height, and it measures half of each triangle’s base length.
The tetrahedron is also a pyramid, but not all pyramids are platonic solids. For example, an obelisk does not qualify because it has an apex above its base rather than parallel to it; nor does a cone qualify because it has an apex that is always above its base rather than parallel to it (the only exception being when it is spun upside down).
Last modified: September 6, 2022