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Two epimers are diastereomers with opposite configurations at exactly one of the two or more tetrahedral stereocenters in their molecules. Their connectivity is the same, and configurations at the other centers agree. The definition is more specific than simply calling two compounds stereoisomers.^[ref]

D-glucose and D-mannose illustrate the relationship: their configurations differ at carbon 2 while the other corresponding stereocenters agree. This local change can alter recognition by enzymes without changing the elemental composition or the carbon-chain connectivity.

For a hypothetical molecule with corresponding stereocenters 2 and 3, the configurations $(2R,3R)$ and $(2S,3R)$ differ at one center and can form an epimeric pair. Changing both to $(2S,3S)$ is a different comparison: it does not satisfy the one-center condition. These labels assume the same molecular connectivity and consistently assigned centers.

A molecule with only one stereocenter does not produce an epimer pair merely by reversing that center: its two configurations are enantiomers, whereas the epimer definition requires a diastereomeric relationship. Epimerization names a process that interconverts an epimeric pair; it does not by itself specify the reaction mechanism, rate or equilibrium ratio.

Mannose is an aldohexose, a six-carbon monosaccharide. The commonly discussed biological form, D-mannose, has neutral molecular formula ${\mathrm{C}}_6{\mathrm{H}}_{12}{\mathrm{O}}_6$. Like other sugars, it can have distinct cyclic forms; specifying D configuration does not by itself identify an alpha or beta anomer.^[ref]

A Fischer projection provides an original structural comparison with D-glucose. Reading hydroxyl positions from carbon2 to carbon5, D-glucose has the pattern right–left–right–right, while D-mannose has left–left–right–right. The difference is at carbon2 alone, so they are epimers. Reversing all four stereocenters would be a different operation, producing the corresponding enantiomer.

Changing that one spatial arrangement changes neither the number of carbon atoms nor the total hydrogen and oxygen counts. A molecular-mass measurement therefore cannot, by itself, distinguish the two sugars. An identifying experiment must supply additional structural discrimination, for example through an appropriately interpreted nuclear magnetic resonance spectrum.

The useful distinction is between composition and configuration. The D label concerns a reference stereocenter; alpha/beta labels concern the anomeric center in cyclic forms. Neither label should be read as a direct statement about sweetness or the sign of optical rotation.^[ref]