The more stable chair conformation of trans-1,2-dimethylcyclohexane has the two methyl groups in the equatorial position. Contrary to the case of methylcyclohexane, which has no interactions in the chair conformation having an equatorial methyl group, the diequatorial conformer of trans-1,2-dimethylcyclohexane has a gauche butane interaction (red and blue carbon atoms) between the two methyl groups. This situation does not arise in 1,3- and 1,4-disubstituted chair cyclohexanes. View the gauche butane interaction (0.9 kcal/mol) as a Newman projection. The other chair conformation of trans-1,2-dimethylcyclohexane has the two methyl groups axial. Recall that a chair-chair conformational inversion interchanges equatorial and axial substituents. The diaxial conformer is equivalent to two axial methylcyclohexanes....one above the ring; one below. Thus there are four gauche butane interactions. Another way to view gauche butane interactions is to count the number of axial methyl groups (or any other group for that matter) that bear a 1,3-relationship to axial hydrogens. The energy of the diaxial conformation is 4 x 0.9 or 2 x 1.8 = 3.6 kcal/mol. Thus, the diaxial conformation is 2.7 kcal/mol (3.6 - 0.9) higher in energy than the diequatorial conformation.