English: Newman projections of butane conformations & their relative energy differences (not total energies). Conformations form when butane rotates about one of its single covalent bond. Torsional/dihedral angle is shown on x-axis.

Conformations (according to IUPAC):

A: antiperiplanar, anti or trans

B: synclinal or gauche

C: anticlinal or eclipsed

D: synperiplanar or cis

Valleys of the pink graph are conformations lowest in energy (shown as A & B). Peaks are conformations highest in energy (shown as C & D). Energies are A<B<C<D with D highest & A lowest in energy. A is thus the most stable conformation &, of all the other conformations, occurs most often in room temperature.

Valleys A & B are local energy minima & A is global minima. A & B can thus be classified as rotamers (a class of conformers). Peaks are not rotamers, & are caused by repulsive forces of the hydrogens & methyls (-CH3).

Source for conformation names & conformer classification:

Pure & Appl. Chem., Vol. 68, No. 12, pp. 2193-2222, 1996

Source for the relative energy differences & other info:

McMurry JE (2003). Organic Chemistry. (8 ed.). Brooks Cole. p. 98. ISBN 9780840054449.