In a free metal ion, all five d-orbitals are degenerate. In an octahedral field, ligands approach along x, y, z axes. Orbitals pointing directly at ligands (dx2-y2 and dz2) experience greater repulsion and go higher in energy — these form the eg set. Orbitals pointing between axes (dxy, dyz, dxz) are stabilised — these form the t2g set. The energy gap is $Delta_{oct}$. Barycentre (average energy) is maintained: t2g is stabilised by 0.4 $Delta_{oct}$ and eg is destabilised by 0.6 $Delta_{oct}$. This splitting explains colour, magnetism, and stability of complexes.