On a horizontal straight line, choose a point O as origin and a point A, to the right of O, such that |OA| is equal to 1 unit. Each positive real number x can then be uniquely represented by a point on the line to the right of O, whose distance from O equals x units, and each negative number by a point on the line to the left of O. The origin represents zero. The line is called the real line when its points are taken in this way to represent the real numbers. Compare Argand diagram.