Please note that the content of this book primarily consists of articles available from Wikipedia or other free sources online. In geometry, the de Longchamps point of a triangle is the reflection of its orthocenter about its circumcenter. A triangle is one of the basic shapes of geometry: a polygon with three corners or vertices and three sides or edges which are line segments. A triangle with vertices A, B, and C is denoted triangle ABC. In Euclidean geometry any three non-collinear points determine a unique triangle and a unique plane (i.e. a two-dimensional Euclidean space).