The set of all possible positions r and momenta p is called the phase space of the system; in other words a set of three coordinates for each position coordinate x, y, z, and three more for each momentum component p x, p y, p z. The entire space is 6- dimensional : a point in this space is (r, p) = (x, y, z, p x, p y, p z), and each coordinate is parameterized by time t. The small volume ("differential volume element ") is written d 3 r d 3 p = d x d y d z d p x d p y d p z .