substitutability

Substitutability is the degree to which two goods, services, or inputs can be used in place of one another to satisfy a need or to achieve a production objective. In consumer theory it refers to how the demand for one good responds to changes in the price of another: if a rise in the price of one good leads to increased consumption of the other, the goods are substitutes. Substitutability is typically measured by cross-price elasticity of demand: a positive value indicates substitutability, a negative value indicates complementarity, and the magnitude reflects the strength of the substitution.

Substitutes range from perfect substitutes, where indifference curves are straight and consumers are indifferent between a fixed rate of substitution, to imperfect substitutes, where substitution varies along the indifference curve. The degree of substitutability depends on technology, preferences, income, and the time horizon.

In production, inputs can substitute for one another in a production function. The marginal rate of technical substitution (MRTS) measures how much of one input can replace another while keeping output constant. The elasticity of substitution summarizes how responsive the input mix is to relative price changes; it is central in models with constant elasticity of substitution (CES).

Substitutability has implications for competition, pricing, and welfare. Greater substitutability tends to intensify competitive pressure and can limit market power when substitutes are readily available. It matters in policy contexts such as antitrust, industrial organization, and energy markets. Real-world examples include butter and margarine, tea and coffee, and various energy sources seeking to replace one another.