What do a refinery manager, a power plant operator, and a gold trader have in common? They all watch the difference between two prices more closely than either price alone. That difference — the spread — often behaves in a stable, predictable way even when the individual prices are all over the place. In this chapter, we’ll learn why those spreads tend to snap back to a long-run average, how to model them, and how to turn that behaviour into pricing tools and trading insights.