Studies on the glass structure of alumino silicates with magic-angle spinning nuclear magnectic resonance (MAS-NMR) techniques  and molecular dynamic simulations (MD)  and  show that RGB-286638 the aluminum is mostly incorporated into the glass network as [AlO4]− tetrahedra which act as network forming species. The negative charge of [AlO4]− tetrahedra is compensated by positively charged cations. As the formation of Al–O–Al linkages is energetically less favorable than Si–O–Al linkages, these Al–O–Al linkages scarcely occur  (Al/Al avoidance principle). The principle is put into perspective by investigations on Si/Al ratios much smaller than unity . In peralkaline or metaluminous compositions, the ratio of aluminum to network modifier is smaller than 1. For these glasses anaphylactic shock is assumed that all aluminum units form tetrahedra with 4/2 bridging-oxygen. The remaining concentrations of network modifier ions form non-bridging-oxygen sites by splitting up the Si–O–Si bridges. Hence, the average number of bridging-oxygen per network forming [SiO4] and [AlO4]− tetrahedron BO/T can directly be calculated from the chemical composition and is a simple measurement of the connectivity and rigidity of the glasses.