Local correlation has been intensively studied, leading to the conclusion that dynamic correlation can be well approximated using domains of localized virtual orbitals that are in the same spatial region as a localized occupied orbital. This reduces the 4th rank tensor of pair correlation amplitudes to an asymptotically linear number of significant elements. Nevertheless, all virtual orbitals are required for post-SCF methods such as coupled cluster theory that recover dynamic correlation, rather than just the much smaller set of valence virtual orbitals. By contrast, static or strong correlation, resides mostly in the valence virtuals (i.e. the antibonding orbitals). Thus complete active space (CAS) methods that seek to describe strong correlation require only a description of the valence virtuals. Methods in this class include CASSCF, 2,39-42 spin-coupled valence bond (VB), and approximations such as generalized valence bond (GVB), coupled cluster valence bond (CCVB), etc.