Comparison of results using synchrotron radiation and X-ray tubes requires a knowledge of the fundamentally different profile shapes inherent in the methods. The varying asymmetric shapes and peak shifts in focusing geometry limit the accuracy and applications of the method and their origins are reviewed. Most o f the focusing aberrations such as specimen displacement, flat specimen and θ-2θ mis-setting do not occur in the parallel beam geometry. The X-ray optics used in synchrotron parallel beam methods produces narrow, symmetrical profiles which can be accurately fit with a pseudo-Voigt function, They have the same shape in the entire pattern. Only the width increases as tanθ due to wavelength dispersion but with higher resolution systems dispersion can be eliminated. The constant instrument function contribution to the experimental profile shape is an important advantage in studies involving profile shapes, e.g., small particle sizes and microstrains, and accurate integrated intensities. The absence of systematic errors leads to more precise lattice parameter determinations.