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The heterosexual–homosexual continuum or the sexual orientation continuum is a model of human sexuality that treats sexual orientation as a continuous spectrum between exclusive heterosexuality and exclusive homosexuality.

Early Models
An early example of such a characterisation is Ulrichs' 1864 Taxonomy of Uranismus, although Ulrichs' taxonomy talked primarily about male sexuality and addressed gender identity and gender variance as much as sexual orientation. Magnus Hirschfeld subsequently proposed a more quantified scheme in 1896.

Other models include the Kinsey scale and Klein grid. Kinsey in particular found various shades of bisexuality to be more common than expected. Klein expanded Kinsey's one dimensional scale to a multi-dimensional grid.

Relation to Gender Identity
Gender binary Postgenderism

Yoruba were agender in pre-colonial period according to somebody whose name I can't remember right now. Lots of non-Europeanised cultures used additional genders instead of distinct labels for sexual orientation.

Difficulties
It is unclear, particularly in ealier models such as the Kinsey scale, precisely what is being measured and how. There is a distinction between sexual orientation, behaviour, and identity. Klein sought to address this with his grid, and R. L. Sell developed another broad profiling technique in the 1990s. Savin-Williams noted that researchers drawing the definitions of sexuality too narrowly around any one aspect usually prevents accurate identification of the target group, and Laumann et al found a lot of inconsistency between different aspects.

Bisexual erasure