Supply and demand

Price set below equilibrium price

Firms would notice the shortages and raise prices to take advantage

Price set above equilibrium price

Firms would notice the unsold stock and reduce the price to shift it

Causes of increase in demand 

An increase in price of substitute goods 

A decrease in price of complements 

An increase in income 

The good becomes fashionable 

Result of increase in demand 

Both price and quantity sold increases  

Causes of decrease in demand

Decrease in price of substituent goods

Increase in price of complements

Decrease in income

Good becomes less fashionable

Result of decrease in demand

Both price and quantity sold decreases