Conformers of Butane

The structure on the left represents the lowest energy conformation of butane, called the anti conformer. The second structure from the left is an eclipsed conformation, and it is a local energy maximum. The third one from the left is the gauche conformer, and it is a local energy minimum. The structure on the right is another eclipsed conformer, and it is at the global energy maximum because the two largest groups are eclipsed. Interconversions between any and all of these conformers are possible.

The rotation animation may help you visualize how the anti conformer can be transformed into the eclipsed conformer. Be aware, however, that this animation greatly simplifies how the atoms really move. The more realistic animation of molecular motion is much more complex.