Constitutional isomers have the same molecular formula, such as C₈H₁₆, but differ in the way the atoms are connected. Constitutional isomers have different numbers in their names or completely different names.

Configurational isomers (or stereoisomers) not only have the same molecular formula, but also the same atom connections. Their names are the same, including numbers for substituents. They differ only in prefixes such as cis, trans, (Z), (E), (R), or (S). If two compounds are configurational isomers they must be either enantiomeric (mirror images of each other) or diastereomeric (not mirror images of each other).