A demand curve is also called a demand schedule. It is the amount demanded at each price. It slopes downward when price is drawn on the y axis and quantity demanded is on the x axis. When a demand curve is drawn we are implicitly holding several other quantities constant: such as, the total income of consumers of the good; the price of substitutes and complements; and consumer preferences. For instance, if we draw the demand curve for oil and the price of natural gas rises (a substitute), then the quantity of oil demanded at each price will rise. That is the same as saying that the demand schedule of oil will shift to the right.