COORDINATE GEOMETRY Two real number lines that are perpendicular to each other and that intersect at their respective zero points define a rectangular coordinate system . Often called the x-y coordinate system or x-y plane . The horizontal number line is called the x-axis and the vertical number line is called the y-axis . The point where the two axes intersect is called the origin , denoted by O. The positive half of the x-axis is to the right of the origin, and the positive half of the y-axis is above the origin. The two axes divide the plane into four regions called quadrants I , II , III , and IV , as shown in the figure below. Each point P in the x-y plane can be identified with an ordered pair (x, y) of real numbers and is denoted by P (x, y). The first number is called the x-coordinate, and the second number is called the y-coordinat...