Take a cuboid unit cell structure where we put an atom on each of the vertices and a different atom in the center. We have a single crystal of that material. Now take one of the cuboid axes to be the ''a'' basis vector. We send in a diffraction beam parallel to that direction. On the other side we will see a diffraction pattern that corresponds to the reciprocal lattice. If we identify each point of this lattice to a set of planes we will find that all these points share on thing. They have ''0'' on the first place. So things like (002). This is according to one of the exercises I have made. However I have a conceptual question - a plane having a vector with a zero on the first place means that the normal vector of this plane always perpendicular to ''a'' or the direction of the incoming beam here. How can you have diffraction happening on planes that are parallel to the incoming beam? In Bragg law you need to always have an angle between the planes and the incoming beam. I'm clearly misunderstanding something big here so please some help.