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Triangle
From Latin: tri- "three" , angulus "corner, angle."
A closed figure with three sides which are straight line segments linked end-to-end.
A 3-sided polygon.
Try this Drag the orange dots on each vertex to reshape the triangle.
(If there is no image below, see support page.)

Triangle properties

Vertex The vertex (plural: vertices) is a corner of the triangle. Every triangle has three vertices.
Base The base of a triangle can be any one of the three sides, usually the one drawn at the bottom. You can pick any side you like to be the base. Commonly used as a reference side for calculating the area of the triangle. In an isosceles triangle, the base is usually taken to be the unequal side.
Altitude The altitude of a triangle is the perpendicular from the base to the opposite vertex. (The base may need to be extended). Since there are three possible bases, there are also three possible altitudes. The three altitudes intersect at a single point, called the orthocenter of the triangle. See Orthocenter of a Triangle.
In the figure above, you can see one possible base and its corresponding altitude displayed.
Median The median of a triangle is a line from a vertex to the midpoint of the opposite side.
Area See area of the triangle and Heron's formula
Perimeter The distance around the triangle. The sum of its sides. See Perimeter of a Triangle
Terminology
It is usual to name each vertex of a triangle with a single capital (upper-case) letter. The sides can be named with a single small (lower case) letter, and named after the opposite angle. So in the figure on the right, you can see that side b is opposite vertex B, side c is opposite vertex C and so on.
Alternatively, the side of a triangle can be thought of as a line segment joining two vertices. So then side b would be called
AC
. This is the form used on this site because it is consistent across all shapes, not just triangles.
Types of Triangle
Triangles are classified into the following groups, depending on various properties. Note that a given triangle can be in more that one group. For example, it could be both a right triangle and a scalene triangle at the same time.
Right Triangle One angle 90°.   See Right triangle definition
Isosceles Two sides equal   See Isosceles triangle definition
Scalene No sides equal   See Scalene triangle definition
Equilateral All sides equal   See Equilateral triangle definition
Obtuse One angle greater than 90°   See Obtuse triangle definition
Acute All angles less than 90°  See Acute triangle definition
Properties
These are some well known properties of triangles. See the section below for a complete list

Related triangle topics

General

Triangle types

Triangle centers

Congruence and Similarity

Triangle quizzes and exercises

(C) 2007 Copyright John Page