The forgotten history of de Broglie waves as themselves artifacts of a Lorentz transform, not physical lengths and frequencies to be transformed, causes confusion for students and others. In this paper the de Broglie wavelength is derived and dependence of de Broglie frequency on velocity explained in terms of Einstein synchronized reference frame measurements of a single clock ("2-on-1") versus measurements of an Einstein reference frame by an observer with a single clock ("1-on-2"). A new memory aid for the relativity of simultaneity, "leading clocks lag," is introduced that puts it on a par with the heuristics "time dilation" and "length contraction" for quick and easy problem visualization.