  We introduce a new quantity for describing nonclassicality of an arbitrary
optical two-mode Gaussian state which remains invariant under any global
photon-number preserving unitary transformation of the covariance matrix of the
state. The invariant naturally splits into an entanglement monotone and local
nonclassicality quantifiers applied to the reduced states. This shows how
entanglement can be converted into local squeezing and vice versa. Twin beams
and their transformations at a beam splitter are analyzed as an example
providing squeezed light. An extension of this approach to pure three-mode
Gaussian states is given.
