I'm trying to learn crystallography and I've had trouble with this concept since the very beginning of the course. It's been so long since it's been introduced that I'd be embarrassed to ask the prof. Right now, I seem to understand the principles of diffraction based on the Miller model; that is, based on the phase changes of radiation scattered from Miller planes at non-n*λ spacings. However, I don't understand what these planes actually are in real life. For a diffraction grating, I can physically see the ridges on the grating, which "validates" the model for me. However, for something as complicated and non-plane-like as a protein, I completely fail to see where the planes originate. Could someone please give me some help?