A two dimensional histogram consists of a set of bins which count the number of events falling in a given area of the @math{(x,y)} plane. The simplest way to use a two dimensional histogram is to record two-dimensional position information, @math{n(x,y)}. Another possibility is to form a joint distribution by recording related variables. For example a detector might record both the position of an event (@math{x}) and the amount of energy it deposited @math{E}. These could be histogrammed as the joint distribution @math{n(x,E)}.