Indifference curves describe our preferences as we evaluate giving up some of one good for another when giving up some of one good for another. Along the indifference curve shown below, the consumer is equally happy consuming all the combinations of good Y and good X traced out by the indifference curve. So, the consumer is just as happy consuming 6 of Y and 1 of X (point A) or consuming 4 of Y and 2 of X (point B) or 3 of each X and Y (point C). The indifference curve incorporates information on the marginal utility from each good. To keep total utility the same, the marginal utility lost when giving up one good must equal the marginal utility gained from the amount gained from the other good.

Y 6 A 5 4 -clad B 3 2 -1 D X 0 2 3 4 5 6