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Official D Blog from February 10, 2019:
Writing a D Wrapper for a C Library by
Several forms ( vitamers ) of vitamin D exist. The two major forms are vitamin D 2 or ergocalciferol, and vitamin D 3 or cholecalciferol; vitamin D without a subscript refers to either D 2 or D 3 or both. These are known collectively as calciferol.  Vitamin D 2 was chemically characterized in 1931. In 1935, the chemical structure of vitamin D 3 was established and proven to result from the ultraviolet irradiation of 7-dehydrocholesterol.