Mathematically, in the sense of geometric topology, each closed surface can be composed of a directed polygon with an even number of sides, and its sides are bonded in pairs. Such a polygon is called a basic polygon.

There are at least three line segments that make up a polygon, and a triangle is the simplest polygon.

Each line segment that constitutes a polygon is called an edge of the polygon; The common endpoint of two adjacent line segments is called the vertex of the polygon; The angle formed by two adjacent sides of a polygon is called the inner angle of the polygon; The line segment connecting two nonadjacent vertices of a polygon is called the diagonal of the polygon.