The Semitic letter Dāleth may have developed from the logogram for a fish or a door. There are many different Egyptian hieroglyphs that might have inspired this. In Semitic, Ancient Greek and Latin, the letter represented /d/ ; in the Etruscan alphabet the letter was superfluous but still retained (see letter B ). The equivalent Greek letter is Delta, Δ .
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.