This article is about the basic geometric shape. For other uses, see Triangle (disambiguation).

Shape with three sides

Equilateral triangle |
---|

A regular triangle |

Type | Regular polygon |
---|

Edges and vertices | 3 |
---|

Schläfli symbol | {3} |
---|

Coxeter diagram | |
---|

Symmetry group | Dihedral (D_{3}), order 2×3 |
---|

Internal angle (degrees) | 60° |
---|

Dual polygon | Self |
---|

Properties | Convex, cyclic, equilateral, isogonal, isotoxal |
---|

Triangle |
---|

A triangle |

Edges and vertices | 3 |
---|

Schläfli symbol | {3} (for equilateral) |
---|

Area | various methods; see below |
---|

Internal angle (degrees) | 60° (for equilateral) |
---|

Triangle = Tri (three) + Angle

A **triangle** is a polygon with three edges and three vertices. It is one of the basic shapes in geometry. A triangle with vertices *A*, *B*, and *C* is denoted $\mathrm{\u25b3}ABC$.^{[1]}

In Euclidean geometry, any three points, when non-collinear, determine a unique triangle and simultaneously, a unique plane (i.e. a two-dimensional Euclidean space). In other words, there is only one plane that contains that triangle, and every triangle is contained in some plane. If the entire geometry is only the Euclidean plane, there is only one plane and all triangles are contained in it; however, in higher-dimensional Euclidean spaces, this is no longer true. This article is about triangles in Euclidean geometry, and in particular, the Euclidean plane, except where otherwise noted.