A combination of high real-space resolution neutron diffraction and 27Al and 29Si NMR experiments will be used to measure the structure of glassy MgO-Al2O3-SiO2. A focus will be on compositions along cuts across the so-called charge-balance line in the ternary phase diagram, where the MgO:Al2O3 ratio along a cut is varied at constant SiO2 content. The strategy is aimed at maximising the change to the Mg-O coordination number which can vary from 4 to 6, i.e., Mg has an ability to transform its identity from a network former to a network modifier. The results will give a complete picture for the inter-dependence of the local Mg, Si and Al coordination environments for a model system that is commonly encountered in glass engineering and in the earth sciences.