Two goods are perfect substitutes when the marginal rate of substitution of one good for the other is constant. The consumer can perfectly substitute apple juice with orange juice. This means that a perfect substitute is a good that functions just the same as the good it is being compared to. Some examples are: Coke vs Pepsi, BP Oil vs Exxon Oil, etc. The indifference curve for a perfect substitute would have no convexity because it makes no difference for consumers.