An R or S configuration is a label specifying the absolute configuration at a stereogenic carbon. After ranking the four groups by the Cahn–Ingold–Prelog priority rules, the lowest-priority group is pointed away from the viewer; if the remaining priorities 1→2→3 then run clockwise the centre is R, and if anticlockwise it is S. This matters because R and S give an unambiguous, drawing-independent way to name each enantiomer.

R 或 S 构型是用来标明手性碳绝对构型的标记。按卡恩–英戈尔德–普雷洛格优先次序规则对四个基团排序后,使最低优先级基团朝远离观察者的方向;若其余优先级 1→2→3 呈顺时针,则该中心为 R,呈逆时针则为 S。这一点之所以重要,是因为 R 和 S 提供了一种不依赖画法、毫无歧义地命名每个对映异构体的方法。