Magnesium diborides superconducting properties were discovered in 2001. Its critical temperature (T') of  is the highest amongst conventional superconductors. Among conventional (phonon-mediated) superconductors, it is unusual. Its electronic structure is such that there exist two types of electrons at the Fermi level with widely differing behaviours, one of them (sigma-bonding) being much more strongly superconducting than the other (pi-bonding). This is at odds with usual theories of phonon-mediated superconductivity which assume that all electrons behave in the same manner. Theoretical understanding of the properties of MgB has nearly been achieved by modelling two energy gaps. In 2001 it was regarded as behaving more like a metallic than a cuprate superconductor.