What is the menaing of "standing plane waves"? Hi All, For me, English is my second language and I am not good is waves also. Here is a passage from QM: "For all practical boundary conditions, the waves must be standing plane waves of all possible wavelengths and amplitudes" What is the significant of saying "standing plane waves"? Author could have waves or plane waves but I think he is trying to convey by saying "standing plane waves". Any help will be appreciated.