If, at a given quantity, the highest price that buyers are willing to pay is equal to the lowest price that sellers are willing to accept, we say the market has reached its equilibrium quantity. Alternatively, when the quantity that buyers are willing and able to purchase at a given price is just equal to the quantity that sellers are willing to offer at that same price, we say the market has discovered the equilibrium price. So equilibrium price and quantity are achieved simultaneously, and as long as neither the supply curve nor the demand curve shifts, there is no tendency for either price or quantity to vary from their equilibrium values.