In modern solid-state physics, it is common to classify systems according to the kind of degrees of freedom they have available, like electron (metals) or spin (magnetism). In crystals that can display the JTE, and before this effect is realised by symmetry-breaking distortions, it is found that there exists an orbital degree of freedom consisting of how electrons occupy the local degenerate orbitals. As initially discussed by Kugel and Khomskii, not all configurations are equivalent. The key is the relative orientation of these occupied orbital, in the same way that spin orientation is important in magnetic systems, and the ground state can only be realised for some particular orbital pattern. Both this pattern and the effect giving rise to this phenomenon is usually denominated orbital-ordering.