where X takes on value is small X within that region that's B X comma Y and then the fraction of the surface it takes up that's the conditional we can also rewrite this as the probability of X comma Y is the probability of Y times probability of x given Y this will become very useful many many situations because let's say we know the distribution for Y and want to know something about X which might be the distribution at the next time slice and if we notice in about X given Y this may be the dynamics of the system to go from