  An approximate formula connecting the true and the HBT homogeneity regions in
multiparticle production processes is derived. It implies that when calculating
the HBT radii one should use the ceter of mass systems of the pairs rather than
the now popular LCMS system. A discussion of several examples clarifies the
potential and limitations of the HBT method. The even cumulants of the
X-distribution, inculding the HBT radii, can be determined for each homogeneity
region, but the relative positions of the homogeneity regions are
unconstrained. This makes the HBT radii of little use for calculating
quantities dependent on the interparticle interactions in coordinate space.
