Certain pairs of molecules differ from one another only in the fact that one is the mirror image of the other. These molecules are known as enantiomers (from the Greek word for "opposite"). The relationship between two enantiomers is the same as the left hand to the right; in fact, you could think of your two hands as enantiomers. By analogy, one enantiomer is called the R-isomer (from the Latin rectus, meaning right), and the other, the S-isomer (from the Latin sinister, or left). Although enantiomers are identical in most of their chemical and physical properties, they can exhibit profound differences under certain conditions, particularly in biological systems.

Molecules that differ from their mirror images are said to be chiral (from the Greek word cheir for "hand"); those that are identical to their mirro images are said to be achiral. The easiest way to tell whether a molecule...