One of the earliest sexual orientation classification schemes was proposed in the 1860s by Karl Heinrich Ulrichs in a series of pamphlets he published privately. The classification scheme, which was meant only to describe males, separated them into three basic categories: dionings, urnings and uranodionings. An urning can be further categorized by degree of effeminacy. These categories directly correspond with the categories of sexual orientation used today: heterosexual, homosexual, and bisexual. In the series of pamphlets, Ulrichs outlined a set of questions to determine if a man was an urning. The definitions of each category of Ulrichs' classification scheme are as follows:

Who created on of the earliest sexual classification schemes?