In geometry, three or more lines are said to be concurrent if they intersect at a single point. Four basic types of concurrent lines are altitudes, angle bisectors, medians, and perpendicular bisectors: Table of Geometry, from the 1728 Cyclopaedia. ... A line, or straight line, can be described as an (infinitely) thin, (infinitely) long, perfectly straight curve (the term curve in mathematics includes straight curves). In Euclidean geometry, exactly one line can be found that passes through any two points. ... The term intersection can mean: a road junction, where two roads intersect each other, such as a roundabout intersection; in mathematics, the set in which two or more other sets intersect each other; see intersection (set theory); a movie; see Intersection (movie). ... A spatial point is an entity with a location in space but no extent (volume, area or length). ...

-In a triangle, altitudes run from each vertex to the point perpendicular on the opposite line. The point where three altitudes meet is the orthocenter.

-Angle bisectors are rays running from the bisector of each angle of the triangle. The all meet at the incenter.

-Medians connect the vertexes in a triangle to the midpoint of the opposite side. They meet at the centroid.

-Perpendicular bisectors are lines running out of the midpoint of each side in a triangle at 90 degree angles. They meet at the circumcenter.

Compare to collinear. Concurrency is the dual of collinearity. A line, or straight line, is, roughly speaking, an (infinitely) thin, (infinitely) long, straight geometrical object, i. ... Duality in the projective plane refers to the interchangeability between points and lines which preserves incidence properties. ...