An expansion of causal distributions is proposed which shows explicitly the structure of these distributions in the « equal-time » limit. It is shown that such an expansion always exists for matrix elements of a commutator in a local axiomatic field theory. The expansion is suitable for the study of the range of applicability of the manipulations usually performed with the equal-time commutators (like construction of field-charge commutators and differentiation of retarded commutators). Furthermore, we show how one can extend these manipulations to cases where equal-time commutators (in the usual sense) do not exist. The expansion is used to study some of the restrictions which are implied by locality on the structure of current commutators.