we care about and sensory measurements so I want to somehow talk about the joint distribution between the two so if we measure one of them we can say something about the other one or we'll deal with dynamics of the world and so we want to relate the distribution now with the distribution at the next time and so both of these involved joint distributions over two random variables rather than just looking at a single variable all right so the joint distribution over x and y is denoted this way then in simple scenarios