How a budget line turns prices into choices
Imagine someone deciding how many coffees and lunches to buy with a set amount of money. Put coffees on one axis and lunches on the other. Each point on the graph represents a possible combination. The budget line traces the combinations that use exactly the available money; points below it leave some money unspent, while points above it cost too much.
The line’s slope captures the tradeoff between the two goods. If one lunch costs as much as three coffees, buying a lunch means giving up the money that could have bought three coffees. That relative price determines how sharply the line tilts. Moving along it does not make the person richer—it changes what they buy while keeping spending at the limit.
When the boundary moves
More income pushes the line outward, making additional combinations affordable. A price change instead pivots the line around the point for the good whose price did not change: if lunches become more expensive, fewer lunches fit into the same budget, but the maximum number of coffees remains the same.
The graph describes what a person can afford, not what they will choose. Preferences, needs, and priorities determine which affordable combination they actually pick. Its practical insight is that every purchase has an alternative: even when money is fully spent, the cost includes the other things that the same dollars could have bought.
