TETRA-PLUS-ONE is a term used to describe a five-sided polygon, also known as a pentagon. It is made up of four sides, PLUS ONE additional side, hence the name tetra-PLUS-ONE. A pentagon is a two-dimensional shape with five straight sides and five angles. It is a regular polygon, meaning all sides are equal in length and all angles are equal in measure. The interior angles of a pentagon add up to 540 degrees. Pentagons are found in nature, such as in the shape of a starfish, and are also used in architecture, such as in the design of the Pentagon building in Washington, D.C.