The short-run market supply in a perfectly competitive market is the horizontal summation of the firms' marginal cost curves when

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In a perfectly competitive market, the firms are price takers and the market price is determined by the intersection of market demand and market supply. In the short run, the firms can adjust their output by changing their level of variable inputs, but the fixed inputs remain constant. Therefore, the short-run supply curve of an individual firm in a perfectly competitive market is its marginal cost curve above its average variable cost curve. The short-run market supply is obtained by horizontal summation of the individual firms' supply curves. The assumption of constant input prices allows the firms' marginal cost curves to be added horizontally to obtain the market supply curve. If input prices vary with changes in industry output, then the marginal cost curves of the firms would shift, and the market supply curve would not be able to be obtained by simply adding the individual firms' marginal cost curves horizontally.

The Short-Run Market Supply in a Perfectly Competitive Market Is