when are two goods said to be substitutes of each other
Substitute good A substitute goods is one goods that can be used instead of another. In consumer theory, substitute goods or substitutes are products that a consumer perceives as similar or comparable, so that having more of one product makes them desire less of the other product. Formally, X and Y are substitutes if, when the price of X rises, the demand for Y rises.
Law of demand Generally the amount demanded of a good increases with a decrease in price of the good and vice versa. In some cases, however, this may not be true. There are certain goods which do not follow this law. These include [Veblen goods] and Giffen goods. Further exception and details are given in the sections below.