For a structure containing the grouping $\ce{R3C–C(Y)=X}$ (with identical or different groups $\ce{R}$) the conformation in which the torsion angle is such that $\ce{X}$ is antiperiplanar to one of the groups $\ce{R}$, and, in a Newman projection, the double bond $\ce{C=X}$ bisects one of the $\ce{R–C–R}$ angles. In this conformation the bond $\ce{C–Y}$ eclipses one of the $\ce{C–R}$ bonds. The other conformation, in which $\ce{X}$ is synperiplanar to one of the groups $\ce{R}$, is called an eclipsing conformation.