We analyze the relation between the classical part of quantum observables and the distributions representing quantum states and observables on the classical phase space. We determine in which conditions such a relation can be established, and the proper phase-space distribution required for this purpose. DOI: 10.1103/PhysRevA.67.064101 PACS number͑s͒: 03.65.Sq, 03.65.Ca, 03.65.Ta The representation of quantum states and observables by distributions on the classical phase space provides a distinguished tool to analyze fundamental aspects of the quantum physics, specially concerning the fuzzy boundary between the classical and quantum theories. For example, they can be used to determine how nonclassical is a quantum state ͑the so-called nonclassical depth͒ ͓1͔.Some recent works have put forward a decomposition of quantum observables into the sum of classical and nonclassical components ͓2,3͔. This splitting presents surprising and interesting properties concerning quantum fluctuations, uncertainty relations, and other fundamental aspects of the quantum theory ͓2-4͔. In particular, it has been shown that the classical part of the linear momentum ͑with respect to position͒ coincides with a local average of the Wigner function.This connection suggests a natural and fruitful relation between classical parts and classical-like description of quantum physics. In this work we study this correspondence in a more general framework by examining the case of arbitrary observables beyond linear momentum. On the one hand, when we consider arbitrary functions of momentum we find that the classical part is the local average of a phasespace distribution different from the Wigner function. ͑Natu-rally, for linear momentum such a distribution and the Wigner function give the same result.͒ On the other hand, if we consider arbitrary joint functions of position and momentum, we find that no phase-space correspondence of this kind can be established. where is the density matrix of the system. For definiteness, in what follows we focus on the case that the reference observable A is the Cartesian position of a onedimensional system, AϭX, while B will be arbitrary in principle. Incidentally, when AϭX, the classical parts are closely related to the de Broglie-Bohm approach to quantum mechanics, as discussed in Refs. ͓4,5͔. Moreover, when B is the linear momentum P, the classical part becomes proportional to the probability current density, which is a very intuitive result. In particular, for pure states ͉͘where (x)ϭ͗x͉͘ as usual. In Refs. ͓2,3͔ it has been shown that P c X can be suitably related to the Wigner function as a kind of a local expectation value of the form