This entry is about the triangle of Euclidean geometry^{1}. It is defined as a planar (two-dimensional) figure bounded by three straight lines and therefore having three interior angles and three exterior angles. The sum of the interior angles of a triangle equals 180°^{2}. It is a convex shape: if you draw a line from any point in the shape to any other, then the line is within the shape.

Triangles were among the first things studied in geometry. This is partly because they're so simple and partly because many shapes can be reduced to triangles in some way.

**Other Definitions of the Word 'Triangle'**

This article is about the type of triangle defined above. The following definitions also exist:

An instrument of percussion, usually made of a rod of steel, bent into the form of a triangle, open at one angle, and sounded by being struck with a small metallic rod.

A draughtsman's square, often called a setsquare, in the form of a right-angled triangle with the other angles being usually 30 and 60 degrees or 45 and 45 degrees.

A kind of frame formed of three poles stuck in the ground and united at the top, to which soldiers were bound when undergoing corporal punishment; this is no longer used.

A small constellation of stars called 'The Triangle' (Triangulum).

A group of three very bright stars visible from the Northern Hemisphere, the Summer Triangle.

A love triangle refers to a romantic relationship involving three people.

Pascal's triangle is a geometric arrangement of the binomial coefficients in a triangle.

Bermuda triangle: A triangular area of the Atlantic whose apices are Bermuda, Miami, and the Lesser Antilles. Reputed to be the site of numerous mysterious disappearances of planes and ships.

**Classification Of Triangles**

There are two methods of classifying triangles: by the relative lengths of their sides and according to the size of their largest internal angle.

When looking at the lengths of sides:

In an *equilateral* triangle all sides are of equal length. An equilateral triangle is also equiangular, ie, all its internal angles are equal - namely, 60°.

In an *isosceles* triangle two sides are of equal length. An isosceles triangle also has two equal internal angles.

In a *scalene* triangle all sides have different lengths. The internal angles in a scalene triangle are all different.

When looking at the size of the largest internal angle:

A *right-angled *triangle^{3} has one 90° internal angle (a right angle).

An *obtuse* triangle has one internal angle larger than 90° (an obtuse angle).

An *acute* triangle has internal angles that are all smaller than 90° (three acute angles).

**Triangle Definitions**

A *vertex*^{4} refers to one of the points of the triangle, where two sides meet.

Any one of the sides may be considered the *base* of the triangle. The perpendicular distance^{5} from a base to the opposite vertex is called an *altitude*^{6}.

A *perpendicular bisector* of a triangle is a straight line passing through the midpoint of a side and being perpendicular to it.

The *circumcenter* is where three perpendicular bisectors meet in a single point; this point is the centre of the *circumcircle*, the circle passing through all three vertices.

The line segment joining the midpoint of a side to the opposite vertex is called a *median*.

The three medians intersect in a single point, the triangle's *centroid*. This is also the triangle's centre of gravity.

Two triangles are said to be *similar* if the angles of one are equal to the corresponding angles of the other. In this case, the lengths of their corresponding sides are proportional. This occurs, for example, when two triangles share an angle and the sides opposite to that angle are parallel.

In a right-angled triangle the *hypotenuse* is the side opposite the right angle and is the longest side.

**The Mathematics of Triangles**

The mathematics of triangles is called trigonometry. It is a massive topic which will not be covered here. It is, however, covered in two other guide entries: one on the trigonometry of right-angled triangles and one on the trigonometric laws of sines and cosines.

The most famous mathematical theorem about triangles is Pythagoras's Theorem. This states that, in a right-angled triangle, the square of the length of the hypotenuse is equal to the sum of the squares of the other two sides. Using this theorem you can calculate the length of any side of a right-angled triangle, provided you already know the length of the other two sides.

**Some Uses Of Triangles**

If you put three points anywhere in space (not in a straight line) you have exactly one plane that goes through all three. This is unique to three points, any more points and there may not be a plane and any less and there are multiple planes. This is useful as it means that a three-legged stool will be stable no matter how uneven the ground^{7}. It also helps simplify some calculations, for example, those in 3-D graphics.

In 3-D graphics, the surfaces of 3-D objects are broken down into triangles. Small numbers of triangles are used for flat surfaces, while large numbers are used to mould curved surfaces similar to the way a geodesic dome is constructed^{8}. The triangle vertices are computed on an X-Y-Z scale and every one must be recomputed each time the object is moved.

Triangluation is using the trigonometry of sines cosines and Pythagoras's Theorem to find your location assuming you know two angles and a length of one side a triangle. Trilateration is how to work out your exact point in space assuming you know the distance to three (if you are working on a plane) or four (in 3D) points. Triangulation and trilateration are used for many purposes, including surveying, navigation, metrology (the science that deals with measurement), astrometry (the measurement of the positions and motions of celestial bodies), binocular vision and gun direction of weapons. They are the basis on which GPS (Global Positioning System) works.

Triangles have many uses in construction due to their strong and rigid shape. They are the basis of many buildings including bridges, monuments^{9} and domes. The geodesic dome is one of the most stable of geometric forms and is made of many triangles (as tetrahedrons) which distribute stress evenly to all elements of the dome, providing a high strength-to-weight ratio. The dome of the Eden Project is an example of such a geodesic dome, although this dome is of triangles arranged together as hexagons and pentagons.