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Generally, the way to tell people which are the 2 black keys closest to the middle of the piano--and the way I tell all my students--is look for the label; it is located in the center of the keyboard, and the 2 black keys are directly under it.

Even for the youngest students, who cannot yet read, they still know letters--and there is nothing else on the underside of the fall board. The problem, sometimes, is they don't yet understand the meaning of right and left, though they might remember which hand through other cues.

As a young student, I dispaired of ever finding middle C quickly and easily. However, my first teacher told me that my nose would learn to find it. And today, it is among the many things I no longer think about. And I tell my students this, too.