values Y can take on of P X comma Y so the reason that will be so important is because often we'll want to get rid of some variables and move to another variable and the way we're going to be doing that is by constructing a Joint Distribution over those two variables somehow and then something out the 1 we don't want anymore more specifically the way this will typically be used is here we'll say well PX is equal to sum over y and imagine we already had access to the distribution for y and we need to construct a joint because you want to