Let us assume that the six ligands are positioned symmetricallyalong the Cartesian axes, with metal atom at the origin.As the ligands approach, first there is an increase in energy of d-orbitalsrelative to that of the free ion just as would be the case in a sphericalfield.The orbitals lying along the axes $(d_{z^2}$ and $d_{x^{2}-y^{2}}$ ) get repelled more stronglythan $d_{xy}, d_{yz}$ and $d_{zx}$ orbitals which have lobes directed between the axes.The $d_{z^2}$ and $d_{x^{2}-y^{2}}$ orbitals get raised in energy and $d_{xy}, d_{yz} ,d_{zx}$ orbitals arelowered in energy relative to the average energy in the spherical crystalfield. Thus, the degenerate set of d-orbitals get split into two sets : thelower energy orbitals set, $t_{2g}$ and the higher energy orbitals set, eg. Theenergy is separated by $\Delta_o$.