Introduction

It is August, a heatwave hits, and every café within two blocks sells out of iced coffee before noon. By the third day the price on the menu has gone up by a dollar. No manager held a meeting about it — it just happened.

The same pattern shows up everywhere: concert tickets resold above face value the morning they go on sale, gas prices climbing the week before a holiday weekend, used-textbook prices crashing the week after finals. Something is coordinating all of these price changes without anyone in charge.

Supply and demand is the model that explains exactly what that something is. It is the single most-used tool in all of economics — the first diagram every student draws and the last one every economist reaches for.

The model answers two questions: why is the price what it is, and what happens when something changes? Every market — coffee, concert tickets, labor, housing — can be analyzed with the same diagram.

On one side sit buyers. Each buyer has a maximum they are willing to pay, and as the price rises, fewer buyers are willing to purchase. That relationship is the demand curve — quantity demanded falls as price rises.

On the other side sit sellers. As the price rises, producing and selling becomes more attractive, so more is offered for sale. That relationship is the supply curve — quantity supplied rises as price rises.

The price settles where the two sides agree: where the quantity buyers want to purchase exactly equals the quantity sellers want to supply. Economists call this the equilibrium — the resting point of the market.

Economists write demand as a straight line: quantity demanded falls as price rises. The parameter $a$ sets the maximum quantity demanded when the price is zero, and $b$ controls how steeply quantity falls as price increases.

Supply follows the opposite logic: quantity supplied rises as price rises. The parameter $c$ is the supply intercept (it can be negative, meaning sellers need a minimum price before they are willing to supply anything), and $d$ controls how steeply quantity rises with price.

The whole page is about where these two lines cross — the price $P^*$ and quantity $Q^*$ at which $Q_d = Q_s$, and what shifts that crossing point.