Change in quantity demanded is caused by the change in own price of the commodity, other things remaining constant. It has two aspects:
(i) Extension of demand: It occurs when quantity demanded increases in response to a fall in own price of the commodity.
(ii) Contraction of demand: It occurs when quantity demanded decreases in response to a rise in own price of the commodity.
When due to change in its own price, quantity demanded of a commodity changes, it is expressed by different points on the same demand curve. In case of extension of demand, it is downward movement and in case of contraction of demand, it is an upward movement on the same demand curve.
