  The Hanbury-Brown Twiss correlation function for two identical particles is
studied for systems with cylindrical symmetry. Its shape for small values of
the relative momentum is derived in a model independent way. In addition to the
usual quadratic ``side'', ``out'' and ``longitudinal'' terms in the exponent of
the correlator, a previously neglected ``out-longitudinal'' cross term is found
and discussed. The model-independent expressions for the size parameters of the
HBT correlation function are interpreted as lengths of homogeneity of the
source, in distinction to its purely geometrical size. They are evaluated
analytically and numerically for two specific thermal models featuring
collective transverse and longitudinal flow. The analytic expressions derived
allow one to establish qualitatively important connections between the
space-time features of the source and the shape of the correlation function.
New ways of parametrizing the correlation function and a new approach to the
measurement of the duration of the emission process are suggested.
