Regular Boundary Value Problems Associated with Pairs of Ordinary Differential Expressions Selfadjoint extensions of M0 Forms generated by selfadjoint extensions of M0 Hilbert spaces generated by positive selfadjoint extensions of Mo Minimal and maximal subspaces for the pair L M Intermediate subspaces Spectra and eigenvalues Resolvents Eigenfunction expansions for selfadjoint subspaces Semibounded intermediate subspaces Some special cases.