Bragg's Law: { 2d*Sin(θ) = (n)λ } is the condition for constructive interference, with n=a positive integer. By using powder crystals of a given atomic spacing (Graphite: d_{100} = 2.1 [Å] and d_{110} = 1.21 [Å]), And by tuning the WaveLength of the electrons to a certain length, we can "pick" a Bragg scattering angle. While any given powder crystal may be at any 3D orientation, only the ones where the geometry gives the "preferred" angle(s) will constructively interfere strongly. 
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Graphite Target Crystal: d_{inner}=d_{10}=0.213 nm d_{outer}=d_{11}=0.123 nm 