In a model with two factor inputs in production, a curve showing the combinations of inputs that have constant market cost. If firms are acting as price-takers in factor markets, the isocost curve is a straight line, whose slope represents the relative prices of different factors’ services. A profit-maximizing firm will minimize the cost of factors required to produce a given output. When the optimal choice involves strictly positive quantities of all inputs, the input mix corresponds to the point of tangency of the isoquant to the lowest isocost curve.