Assigning priorities to substituents:

The group of lowest priority (the hydrogen) is already located behind the page (note: the hatched bond pointing out the bond going away from you). An arc is now drawn that connecting the remaining groups, beginning from the group of highest priority and finishing at the group of that have 3^rd priority. The assignment is R (rectus), if the arc is drawn clockwise. The assignment is S (sinister), if the arc is drawn counterclockwise. In this instance the arrow is drawn clockwise. Hence, the molecule is (R)-lactic acid.

A 2^nd illustration illustrates another rule involving substituents along with double bonds. The asymmetric center is marked with an asterisk (*). The atoms straight attached to the asymmetric center are displayed on the right with their atomic numbers. At this point it is probable to set the group of highest priority (the oxygen) and the group of lowest priority (the hydrogen). There are two similar carbons that attached to the asymmetric center thus we have to move to the next stage and make out the atom with the highest atomic number joined to each of the similar carbons. This still does not differentiate among the CHO and CH₂OH groups because both carbon atoms comprise an oxygen atom attached. The next stage is to look at the 2^nd most significant atom attached to the two carbon atoms. Though, if there is a double bond present, you are permitted to 'visit' similar atom 2 times. The next most significant atom in the CH₂OH group is the hydrogen. In the CHO group, the oxygen may be 'revisited' because there is a double bond. Hence, this group takes priority over the CH₂OH group.

Figure:   (a) Assigning priorities to substituents; (b) assigning an asymmetric center as R or S.

Figure:   Assigning priority to substituents of an asymmetric center.

Figure:   Assigning priority to substituents of an asymmetric center.

Figure:   Assigning an asymmetric center as R or S.

The priorities have been ascertained, and thus the group of lowest priority is located behind the page and the three most significant groups are linked to see if they are clock- wise or counterclockwise.