This measures how responsive the quantity demanded of one good responds to changes in the price of other goods. The formula, with Q still on top, is as follows:
Cross price elasticity of demand = % change in quantity of good X / % change in price of good Y
Two goods which are substitutes will have a positive cross elasticity. An increase in the price of one good, for example Pepsi, will lead to an increase in the quantity demanded of a substitute, for example Coca Cola.
Two goods which are complements will have a negative cross elasticity. An increase in the price of one good, for example Playstation 3, will lead to a fall in demand of a complement , for example Playstation 3 games.
The cross elasticity of two goods which have no relationship to each other would be 0, for example apples and shoes.