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Allowed transitions observed using electron magnetic resonance (EMR) in transition metal (3dN or 4fN) complexes may be predicted by the selection rules. Non-standard selection rules justified by mixing of wavefunctions emerge for nominally forbidden transitions. We present tutorial guide on selection rules in EMR, optical, and inelastic neutron scattering spectroscopy. This includes basic concepts in quantum mechanics and group theory underlying derivations of selection rules and useful rules for nonzero matrix elements. These rules provide crucial information for practitioners. Survey of usage of selection rules in EMR and related studies of novel systems in emerging areas is provided at an introductory level. Applications to modern case systems include: parallel field EMR excitations for integer spin vs half-integer spin ions, nitrogen vacancies in diamond, Haldane gap systems, exchange coupled systems and molecular nanomagnets, and magnetic adatoms on surfaces. Various misconceptions concerning crucial notions, which constitute terminological confusion, have been clarified. [ABSTRACT FROM AUTHOR]