R/S nomenclature

R configuration	S configuration
spinqualitylabelsall models Carbon (gray) with four substituents (labeled by priority 1-4) shows an R-configuration because when substituent 4 is in the back, substituents 1 - 3 run clockwise. Show:Asymmetric Unit Biological Assembly Drag the structure with the mouse to rotate   Export Animated Image	spinqualitylabelsall models Carbon (gray) with four substituents (labeled by priority 1-4) shows an S-configuration because when substituent 4 is in the back, substituents 1 - 3 run counterclockwise. Show:Asymmetric Unit Biological Assembly Drag the structure with the mouse to rotate   Export Animated Image

R and S are used to describe the stereochemistry at an sp3 carbon center with four different substituents. First, each substituent is assigned a priority. Then, the priority 4 substituent is placed facing away, and the rotation going from 1 to 2 to 3 is considered. If it is clockwise, the configuration is (R), if counterclockwise, (S). You can rotate the model on the left and the right. It is impossible to orient them in a way that they superimpose perfectly. Thus, they are different molecules. If you inspect them closer, you will see that they are like image and mirror image, i.e. they are enantiomers (if we assume that the substituents are all different and are themselves achiral as they appear here).