比如一个酮，是平面结构的，按照顺序规则将氧，R1,R2集团排列，如果这三个集团是顺时针的，则这个面是Re面，若是逆时针的，则这个面是Si面，你可以去找不对称合成的书来看看。

Stereochemistry also plays a role assigning faces to trigonal molecules such as ketones. A nucleophile in a nucleophilic addition can approach the carbonyl group from two opposite sides or faces. When an achiral nucleophile attacks acetone, both faces are identical and there is only one reaction product. When the nucleophile attacks butanone, the faces are not identical (enantiotopic) and a racemic product results. When the nucleophile is a chiral molecule diastereoisomers are formed. When one face of a molecule is shielded by substituents or geometric constraints compared to the other face the faces are called diastereotopic. The same rules that determine the stereochemistry of a stereocenter (R or S) also apply when assigning the face of a molecular group. The faces are then called the re-faces and si-faces. In the example displayed on the right, the compound acetophenone is viewed from the re face. Hydride addition as in a reduction process from this side will form the S-enantiomer and attack from the opposite Si face will give the R-enantiomer. However, one should note that adding a chemical group to the prochiral centre from the re-face will not always lead to an S stereocenter, as the priority of the chemical group has to be taken into account. That is, the absolute stereochemistry of the product is determined on its own and not by considering which face it was attacked from. In the above mentioned example, if chloride (Cl-) was added to the prochiral centre from the re-face, this would result in an R-enantiomer，

FRANCIS A. CAREY的高等有机化学 那本书里有介绍

这一般是对于一个潜手性化合物而言的，你把这三个基团按大小顺序排列如果是顺时针则为Re面，逆时针为Si面，但这不是绝对的，要看你是从哪个方向看的，比如你这面看是Re面，但从反面看就是Si面了

和R，S构型判断方法是一样的