The thick horizontal line in the above diagram is called the **x-axis**, and the thick vertical line is called the **y- axis**. The point at which the axes meet, (0, 0), is called the **origin**. On the x-axis, positive numbers are to the right of the origin and increase in size to the right; further, negative numbers are to the left of the origin and decrease in size to the left. On the y-axis, positive numbers are above the origin and ascend in size; further, negative numbers are below the origin and descend in size. As shown in the diagram, the point represented by the ordered pair (x, y) is reached by moving x units along the x-axis from the origin and then moving y units vertically. In the ordered pair (x, y), x is called the **abscissa** and y is called the **ordinate**; collectively they are called coordinates. The x and y axes divide the plane into **four quadrants**, numbered **I, II, III, and IV** counterclockwise. Note, if x ≠ y, then (x, y) and (y, x) represent different points on the coordinate system.