Triangles can also be classified according to their internal angles, described below using degrees of arc:

A right triangle (or right-angled triangle, formerly called a rectangled triangle) has one 90° internal angle (a right angle). The side opposite to the right angle is the hypotenuse; it is the longest side in the right triangle.
An oblique triangle has no internal angle equal to 90°.
An obtuse triangle is an oblique triangle with one internal angle larger than 90° (an obtuse angle).
An acute triangle is an oblique triangle with internal angles all smaller than 90° (three acute angles).

A triangle is one of the basic shapes of geometry: a polygon with three corners or vertices and three sides or edges.

Properties of Triangles (and some activities for the classroom).
Triangles can be classified according to the relative lengths of their sides:

In an equilateral triangle, all sides are of equal length. An equilateral triangle is also an equiangular polygon, i.e. all its internal angles are equal—namely, 60°; it is a regular polygon.
In an isosceles triangle, two sides are of equal length (originally and conventionally limited to exactly two). An isosceles triangle also has two equal angles: the angles opposite the two equal sides.
In a scalene triangle, all sides have different lengths. The internal angles in a scalene triangle are all different.
equilateral triangle
isosoles triangle
scalene triangle
right - angled triangle
obtuse triangle
acute triangle
Click here! A page of triangles to print. Students measure and label the sides and angles, and say what sort of triangle each one is. (Do not use the words "scalene" or "oblique". The triangle is equilateral, isosolese, right-angled, obtuse or acute.