Before we leave linear relationships, we need to consider two special kinds of linear relationships that come up in economics from time to time. To the right we show a graph of a linear relationship that is a straight, horizontal line. Such a line has a slope of zero. As we move to the right from point **A** to point **B**, **Q** increases from **10** to **24**, but **P** remains unchanged at **12**. So, the rise is **zero**, even though the run is **14**. When we compute **rise ÷ run** for any two points on this graph the answer will always be zero since the rise will always be zero. Therefore, the **Slope = 0**.

This graph shows that **P** and **Q** are not related; **P** remains fixed at a value of **12** regardless of the value of **Q**.