have the backward coming in XT plus 1 the forward coming into XT you're multiplying the conditional and the one observation conditional that you hadn't incorporated yet now you might wonder why would we care about pairwise posterior I shall come clear later in this lecture for now you might say well why would ever care but we'll clarify that alright so these are just the laws apply but the same as we did on the board oh you might take this to the next level a little harder to do you can do as an exercise can I find the joint between XT