Natural numbers and simple geometric figures

A point is the simplest geometric figure and has no length. Through any two points of a plane only one line can be drawn; it goes on forever in both directions. A point on a line splits it into two rays; a ray has a beginning but no end. The part of a line limited by two points is called a segment and has a definite length. Lines that do not intersect are called parallel. The sum of the side lengths of a triangle is called the perimeter of the triangle.