  Diffracted X-ray waves in crystals have relative phases regarding the
mathematical format used to describe them. A forward propagating wave can be
defined with either negative or positive time evolution, i.e. kr - wt or $wt -
kr. Physically measurable quantities are invariant with respect to the choice
of definition. This fact has not been clearly emphasized neither extensively
explored when deriving well-established equations currently being used in many
X-ray diffraction related techniques. Here, the most important equations are
generalized and consequences of conflicting undertaken definitions discussed.
