Supply curve

A supply curve makes a conditional offer relationship visible. The usual diagram places price vertically and quantity horizontally, while the algebra may write quantity supplied as a function of price. The chosen form determines the slope's units.

Movement and shift are different claims

A change in the good's own price selects another point on the same curve. A change in input costs, technology, taxes, expectations, capacity, regulation, or seller population can shift the curve. An observed change in sales does not by itself identify either mechanism because demand may also have changed.

A straight upward-sloping line is a bounded approximation. Some industries have capacity steps, inventories, joint products, network effects, or shutdown and entry decisions that a line omits. A negative algebraic intercept can also be a mathematical extension outside the relevant domain rather than a literal offer.

The curve is not a production ledger, causal estimate, or proof of marginal cost. It is a model whose unit, period, domain, controls, and empirical basis must travel with its use.