R and S configuration

Once you know a stereocenter exists, you still need a name for which of its two mirror arrangements you are looking at — its 'handedness' written down. The R/S system gives every chirality center an unambiguous label, the way 'clockwise' and 'counterclockwise' name the two ways a dial can turn.

Here is the procedure. First, use the Cahn-Ingold-Prelog rules to rank the four groups on the stereocenter from highest priority (1) to lowest (4). Next, orient the molecule so the lowest-priority group (4) points directly away from you, like the tail of an arrow flying off. Now look at the remaining three groups and trace the path 1 to 2 to 3. If that path curves clockwise, the center is R (from Latin rectus, right); if it curves counterclockwise, it is S (sinister, left). One handy shortcut: if the lowest group happens to point toward you instead, determine the rotation and then simply flip the answer.

R and S are 'absolute configuration' — they describe the actual three-dimensional arrangement of a real molecule, independent of how it rotates light. This makes them the universal language of stereochemistry: a journal can specify (S)-naproxen or (2R,3S)-tartaric acid and any chemist worldwide can build exactly the right molecule. Note that R/S is a naming convention and does not by itself predict the sign of optical rotation — an R compound is not automatically dextrorotatory.