Law of Diminishing Marginal Rate of Substitution:

A typical indifference curve also illustrates the law of diminishing marginal rate of substitution.

The less of one good (beer) the consumer gave up and the more of another commodity (apple) he obtained in the process, the less willing he is to give up a unit of the former good (beer) for an additional unit of the latter good (apple).

A careful inspection will confirm the law of diminishing marginal rate of substitution.