When two goods are completely interchangeable, they are

In economics, goods that are completely interchangeable are known as perfect substitutes. These are products that a consumer perceives as identical or so similar that they can be used in place of one another without any loss of utility [3]. For perfect substitutes, the marginal rate of substitution (MRS) remains constant, meaning the consumer is willing to exchange one good for the other at a fixed ratio regardless of how much they possess. This contrasts with imperfect substitutes, where consumers perceive differences in taste or quality [3]. While complements are goods consumed together (like tea and sugar), substitutes like tea and coffee are consumed in place of each other [2]. Giffen and Veblen goods are exceptions to the law of demand where consumption increases with price, but they do not define the relationship of interchangeability between two distinct goods [1].

Sources

1. [2] Microeconomics (NCERT class XII 2025 ed.) > Chapter 2: Theory of Consumer Behaviour > 2.4.4 Substitutes and Complements > p. 25
2. [3] https://www.investopedia.com/terms/s/substitute.asp
3. [1] Microeconomics (NCERT class XII 2025 ed.) > Chapter 2: Theory of Consumer Behaviour > 2.4.3 Normal and Inferior Goods > p. 24