The Law of Demand and the Demand Curve

Quantity demanded is the amount consumers are willing and able to buy at a particular price during a specified period. The law of demand says that, other things equal, a higher price normally reduces quantity demanded and a lower price increases it.

From schedule to curve

Economics graphs put price vertically and quantity horizontally. Connecting the schedule's points produces a downward-sloping demand curve. It represents a set of conditional statements—“if price is this, quantity demanded is that”—not a record of one transaction.

From individual to market demand

At a price of $6, suppose A buys 2 cups, B buys 1, and C buys 3. Market quantity demanded is the horizontal sum, 6 cups. Repeat at each price to construct market demand. Do not add prices or merely count buyers; one buyer may demand several units.

A linear relationship can be written:

$Q_d=60-5P$

At $P=4$, $Q_d=40$. Rearranging gives inverse demand, $P=12-0.2Q_d$, which is convenient because price is vertical. It can also be read as willingness to pay for the marginal unit. Substitution, purchasing-power effects, and diminishing marginal benefit explain why lower prices open additional uses.

Substitution effects, income effects, and diminishing marginal benefit help explain the law. It is a general relationship, not a logical necessity, and relies on the other-things-equal condition.