In economics, what law says that quantity demanded usually falls when price rises?

In economics, the law saying quantity demanded usually falls when price rises is the law of demand.

The law of demand describes an inverse relationship between a good's price and the quantity consumers are willing and able to buy, assuming other factors remain unchanged. A lower price can make a product more affordable and may encourage people to buy more of it. A higher price generally has the opposite effect.

Economists often represent this relationship with a downward-sloping demand curve. The word “usually” matters because unusual cases, such as certain luxury or status goods, can complicate the pattern. A change in the product's own price causes movement along the curve, while changes in income, tastes, expectations, or prices of related goods can shift the entire curve.

The law of demand is different from the law of supply, which concerns sellers and normally predicts that a higher price encourages a greater quantity supplied.