The two-electrons-in-two-orbitals active space model of electronic structure of biradicals and biradicaloids is extended to a Hamiltonian that incorporates the usual kinetic and electrostatic energy terms, and also outside electric and magnetic fields, spin-spin dipolar terms, and one- and two-electron spin-orbit coupling terms. It leads to a more rigorous version of the Salem-Rowland rules for the dependence of T-1-S-0 spin-orbit coupling (SOC) in biradicals and biradicaloids on molecular structure and conformation. For large T-1-S-0 SOC in a bitopic biradical, (i) the localized orbitals A and B that are singly occupied in the T-1 state either interact covalently or one of them is sufficiently lower in energy to have nearly double occupancy in the S-0 state, (ii) on at least one atom of reasonably high atomic number one p orbital contributes strongly to A and another to B, and (iii) the atomic contributions add constructively rather than destructively. The nature of this addition is such that inverse heavy atom effects on SOC are possible. Through-bond coupling is essential and its effects are apparent from simple resonance structures, illustrated on alpha,omega-alkanediyl biradicals. Implications for the zero-field splitting parameters of triplet states are noted.