The law of demand is a ceteris paribus claim

A higher own price reduces quantity demanded, other influences held constant.

A demand schedule lists quantities buyers plan to purchase at different prices. Plotting those pairs produces a demand curve. Its negative slope reflects several mechanisms: buyers substitute toward alternatives, their purchasing power falls when the price rises, and units differ in marginal willingness to pay. The curve does not claim that every consumer buys the same amount or that quantity must fall in every historical episode where price rises. Other determinants can change at the same time.

Demand includes both willingness and ability to pay. Wanting a new laptop without the purchasing power to buy it at any relevant price is not market demand. Likewise, an actual purchase is not the entire demand relationship. It is one realized quantity at one price. A schedule makes the distinction visible by pairing several possible prices with the quantity planned at each one during a stated period.

Every row assumes the nonprice determinants are held constant. The Latin phrase ceteris paribus means “other relevant things equal.” It is not a claim that the real world never changes. It is a method for isolating the response to the good’s own price. If income, preferences, or the number of buyers also changes, the observed outcome can move in a direction different from the fixed-curve prediction.

An own-price change causes movement along demand. If movie-ticket price falls from $14 to $10 and attendance rises, quantity demanded increases. Calling this an “increase in demand” is imprecise on the exam because the curve did not shift.

The wording should preserve cause and effect. A lower ticket price causes a greater quantity demanded. An increase in demand can cause a higher equilibrium price after the curve shifts. Saying “price rises, so demand rises” usually reverses the law of demand or confuses a shift with the new equilibrium. Circle whether the stem changes the good’s own price or something held outside the axes.

The demand curve can also be read as marginal willingness to pay. The height of the curve at a quantity represents the maximum price buyers are willing to pay for that marginal unit. Because later units often serve less urgent uses, willingness to pay declines. This interpretation supports consumer-surplus and efficiency analysis later in the book.

A real observation need not trace one curve

During a heat wave, both the price and sales of bottled water rise. This does not violate the law of demand. Hot weather increases demand, shifting the curve right. Movement to a new equilibrium can raise both price and quantity. The law of demand predicts what quantity would do in response to price on a fixed demand curve, not what two historical observations must show when determinants changed.

Do not use rare exceptions as an excuse to ignore the model. Unless a question explicitly gives prestige effects, Giffen conditions, or another unusual assumption, apply the ordinary downward-sloping demand relationship. The exam tests whether you can use a stated introductory model, not whether you can invent a complication that the stem did not supply.

At the end of a demand item, use one of two precise sentences. For an own-price change: “Quantity demanded rises or falls along the existing demand curve.” For a determinant change: “Demand shifts right or left, so buyers plan a different quantity at every possible price.” The nouns prevent a large family of distractor errors.