Several forms (vitamers) of vitamin D exist. The two major forms are vitamin D or ergocalciferol, and vitamin D or cholecalciferol. Vitamin D without a subscript refers to either D or D, or both, and is known collectively as calciferol.
Vitamin D was chemically characterized in 1931. In 1935, the chemical structure of vitamin D was defined and shown to result from the ultraviolet irradiation of 7-dehydrocholesterol. A chemical nomenclature for vitamin D forms was recommended in 1981, but alternative names remain in common use.
Chemically, the various forms of vitamin D are secosteroids, that is, steroids in which one of the bonds in the steroid rings is broken. The structural difference between vitamin D and vitamin D is in the side chain, which contains a double bond, between carbons 22 and 23, and a methyl group on carbon 24 in vitamin D. Many vitamin D analogues have been synthesized.