where x {\displaystyle x} and y {\displaystyle y} are particular values of X {\displaystyle X} and Y {\displaystyle Y} , respectively, P ( x , y ) {\displaystyle P(x,y)} is the joint probability of these values occurring together, and P ( x , y ) log 2 ⁡ [ P ( x , y ) ] {\displaystyle P(x,y)\log _{2}[P(x,y)]} is defined to be 0 if P ( x , y ) = 0 {\displaystyle P(x,y)=0} .