In this type of projection, the bond between carbon atoms is shown as a long straight line. The lower part of the line classifies the front carbon atom whereas the upper part classifies the rear carbon atom. Since each carbon atom in ethane is connected to three hydrogen atoms; each carbon atom has three lines connected designating C-H bonds elevated at an angle of 120° to each other.
A form of stereoisomerism where interconversions of isomers are possible by rotations referring to single bonds is called as Conformational Isomerism. These isomers are termed as Conformational isomers. In case of single bond rotation, Rotational Energy acts as a barrier Conformational Isomerism to occur, the energy barrier must be a small one. There are numerous types of conformational isomers, such as Ethane and Butane.
As we have come to know, alkanes have a very simple C-C single bond in the higher alkanes. A bond does like this does not exist in methane, but onwards to ethane, all alkanes have a single bond of C-C. This will allow the rotation of these single bonds to form different spatial arrangements of the carbon atoms, which in turn form different conformations of these alkanes.
Now let us consider butane, a little bigger molecule. There are now 3 rotating carbon-carbon bonds to consider, but we will focus on the center bond between C2 and C3. Below are two examples of butane in a conformation which puts the 2 CH3 groups (C1 and C4) in the eclipsed position.