which means that they provide Lilly with the same level of utility. Thus, the marginal utility that Lilly would

gain from, say, increasing her consumption of books from two to three must be equal to the marginal utility

that she would lose if her consumption of doughnuts was cut from 120 to 84—so that her overall utility remains

unchanged between points A and B. Indeed, the slope along an indifference curve as the marginal rate of

substitution, which is the rate at which a person is willing to trade one good for another so that utility will

remain the same.

Indifference curves like Um are steeper on the left and flatter on the right. The reason behind this shape

involves diminishing marginal utility—the notion that as a person consumes more of a good, the marginal

utility from each additional unit becomes lower. Compare two different choices between points that all provide
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