Basis     of Difference      Extension of Demand     Increase in Demand Meaning When quantity demand for a commodity rises due to fall in the price, keeping other factors as constant is called an extension of demand. When demand for commodity increases due to change in other factors, keeping price as constant is called an increase in demand. Cause Fall in price of the  commodity. Several causes: a. Increase in price of the substitute goods. b. Decrease in price of the complementary goods. c. Increase in income in case of normal goods. d. Favorable tastes and preferences are developed. Effect on Demand   curve It is shown as movement from left to right on the same  demand  curve. It is shown as   forward or rightward  shift  in the demand curve. Concept It is a quantity demand  concept.