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In mathematics, a horn angle, also called a cornicular angle, is a type of curvilinear angle defined as the angle formed between a circle and a straight line tangent to it, or, more generally, the angle formed between two curves at a point where they are tangent to each other.

History of geometry
Non-Archimedean geometry

References

Thomas Little Heath, T.L. (1908). The thirteen books of Euclid's Elements 2. The University Press. pp. 39–42.