Hello all, I don't understand some things concerning reciprocal space. I know how it appears from quantum mechanics (it comes from gas of free electrons model, then applying Fermi-statistics for it and solve Heisenberg equation and get some kx,y,z which have to be integer. After that we call the space of these k-values as k-space, or reciprocal [Ashkroft, Mermin for instance]). But I also know that there is another reciprocal space which used for crystallography (i.e. for X-ray). This one comes from Laue equation. Both of these spaces are linked somehow, nevertheless it came from different time and different (a bit) physics. Excuse me for the my more general question which cannot be answered with a few words, but maybe someone experienced can understand my lack of knowledge and guide me. Thanks for your attention.