Lilly's Indifference Curves

An indifference curve joins the combinations of two goods that a person likes equally well. Lilly is just as happy with two books and 120 doughnuts as with three books and 84, or eleven books and 40, or twelve books and 35. All four sit on the middle curve.

The curves slope down because giving up one good has to be paid for with more of the other. They bow toward the origin because the more doughnuts she already has, the less another one is worth to her, so the trade she will accept changes as she moves along. Curves further from the origin mean more of both goods and so more satisfaction: any point on the highest curve beats any point on the middle one. The slider picks out one curve; the toggles show or hide the labelled points.