There is an uncertainty relation between the x component and the y component of the angular momentum L of a particle, because [Lx, Ly] = i[itex]\hbar[/itex]Lz which is not 0. But what happens when Lz does equal 0? Would we in principle be able to measure both the x and y components of the angular momentum with no uncertainty? What kind of state would this describe? I've searched my textbooks and the web for an answer to this question, but I haven't found any, so maybe this is actually something really obvious which I'm not seeing! Thanks for your help!