An isomer which is related to a central atom about which bonded groups may vary in their arrangement so that the molecules are not superimposable. To name a configurational isomer, draw a tetrahedral image so that the least complex attached group is in back of the plane of observation and move from the lowest complexity group remaining to the highest complexity group. If the direction is right, the species is rectus (R). If the direction is left, the species is sinister (S).