In geometry, a base is a side of a plane figure or face of solid, particularly one perpendicular to the direction height is measured or on what is considered to the bottom. This usage can be applied to a triangle, parallelogram, trapezoids, cylinder, cone, pyramid, parallelopiped or frustum. By extension, the length or area of a base is also called a base. As such, bases are commonly used in formulas for area and volume. An example would be a triangular prism; its bases are square.