I would like to know whether the output for XRD is given as 2 theta simply because the x-ray beam and the detector both move through an angle theta while the sample is stationary. The sum of these two angles where there is a maximum gives a signal at 2 theta which in turn can be converted to theta and hence to d (using Bragg equation). Therefore why don't we just get output as theta instead of 2 theta? Is it simply a convention?