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Galactose is an aldohexose, a six-carbon monosaccharide whose open-chain reference form has an aldehyde group. It is the carbon-4 epimer of glucose: the corresponding structures differ at that one stereocenter. Its molecular formula is ${\mathrm{C}}_6{\mathrm{H}}_{12}{\mathrm{O}}_6$.^[ref]

A Fischer projection makes the difference precise. With the aldehyde at the top, the carbon-2 through carbon-5 hydroxyl group pattern of D-glucose is right–left–right–right. Changing carbon 4 alone gives right–left–left–right for D-galactose. Reversing every stereocenter would instead make an enantiomer; the single-position change is an epimerization.

Neither the formula nor an accurate mass measurement distinguishes these two sugars, because moving a hydroxyl group from one side of a stereocenter to the other changes no atom counts. Their different spatial arrangements nevertheless present different binding surfaces to an enzyme. This is a useful example of why carbohydrate identification needs structural information in addition to elemental composition. The D designation fixes the reference stereochemistry; it does not itself select a single cyclic anomer or state the sign of optical rotation.