In the textbook story, favored by the followers of Léon Walras, if the quantity demanded does not equal the quantity supplied in a market, "price adjustment" is the rule: if there is a market surplus or glut (excess supply), prices fall, ending the glut, while a shortage (excess demand) causes price to rise.
Economist Alfred Marshall saw market adjustment in quantity-adjustment terms in the short run.
Quantity adjustment contrasts with the tradition of Léon Walras and general equilibrium.
For Walras, (ideal) markets operated as if there were an Auctioneer who called out prices and asked for quantities supplied and demanded.
In this pure theory, no actual trading was allowed until the market-clearing price was determined.