For every reflection corresponding to a point q in the reciprocal space, there is another reflection of the same intensity at the opposite point -q. This opposite reflection is known as the "Friedel mate" of the original reflection. This symmetry results from the mathematical fact that the density of electrons "f"(r) at a position r is always a real number. As noted above, "f"(r) is the inverse transform of its Fourier transform "F"(q); however, such an inverse transform is a complex number in general. To ensure that "f"(r) is real, the Fourier transform "F"(q) must be such that the Friedel mates "F"(−q) and "F"(q) are complex conjugates of one another. Thus, "F"(−q) has the same magnitude as "F"(q) but they have the opposite phase, i.e., "φ"(q) = −"φ"(q)