  Using two specific models and a model-independent formalism, we show that in
addition to the usual quadratic ``side'', ``out'' and ``longitudinal'' terms, a
previously neglected ``out-longitudinal'' cross term arises naturally in the
exponent of the two-particle correlator. Since its effects can be easily
observed, such a term should be included in any experimental fits to
correlation data. We also suggest a method of organizing correlation data using
rapidity rather than longitudinal momentum differences since in the former
every relevant quantity is longitudinally boost invariant.
