Why do some vitamins have both a letter-number name (e.g., B12) and a chemical name (e.g., cobalamin)?

Numbered vitamin names indicate specific enzyme cofactors while chemical names indicate pro-vitamins only; therefore both names are needed because vitamins always exist in both cofactor and precursor states simultaneously.

Letter-number names identify vitamins by their solubility solely, while chemical names categorize vitamins by their physiological function, and the two naming systems are unrelated historically.

The letter-number is the historical vitamin name reflecting discovery order and the chemical name describes the specific molecular structure; both names refer to the same vitamin but serve different descriptive purposes.

Chemical names are used only for synthetic supplements and letter-number names are reserved for naturally occurring vitamins, so the two systems never refer to the same molecule in practice.