Since the angular momenta are quantum operators, they cannot be drawn as vectors like in classical mechanics. Nevertheless, it is common to depict them heuristically in this way. Depicted on the right is a set of states with quantum numbers ℓ ℓ = 2 , and m ℓ ℓ = − − 2 , − − 1 , 0 , 1 , 2 for the five cones from bottom to top. Since | L | = L 2 = ℏ ℏ 6 , the vectors are all shown with length ℏ ℏ 6 . The rings represent the fact that L z is known with certainty, but L x and L y are unknown; therefore every classical vector with the appropriate length and z -component is drawn, forming a cone. The expected value of the angular momentum for a given ensemble of systems in the quantum state characterized by ℓ ℓ and m ℓ ℓ could be somewhere on this cone while it cannot be defined for a single system (since the components of L do not commute with each other).