A line extends indefinitely in both directions. When a line is plotted there is a point where it meets the y-axis, this is called the y-intercept. At the point where the y-intercept lies the x-coordinate of the line is equal to 0.

Slope is the rate at which the y-coordinate of points lying on a line changes with respect to a change in the x-coordinate of the points. If two points on a line have the coordinates (x1, y1) and (x2, y2) the slope of the line is equal to (y2 - y1)/(x2 - x1).

The slope of a line can be positive, negative, zero or equal to infinite. It is positive if for points on the line the sign of the change in the x-coordinate and that in the y-coordinate is the same; the line is upward sloping in this case. If the line is downward sloping the slope is negative. A line parallel to the x-axis has a slope of zero and a line parallel to the y-axis has a slope of infinity.