Triangles are one of the fundamental figures used in Euclidean geometry. There are three elements required to make a triangle. It is a 3-sided, Plane or two-dimensional figure, in which the sum of the interior angles equals exactly 180 degrees.  There are two common systems of triangle classification. One focuses on the sides and designates three types of triangle.

Equilateral Triangle: Equilateral means “equal sides,” and in an equilateral triangle, all three sides are the same length. This means that the angles will also be equal – all 60° - making the triangle equiangular as well. 

Isosceles Triangle: Isosceles means “equal legs,” and an isosceles triangle has two sides that are equal in length. This also means that the two angles formed where the equal sides meet the third sides are equal. 

Scalene Triangle: Scalene comes from a word meaning "uneven," and a scalene triangle has three unequal sides. As you might suspect, then, the three angles are unequal as well.  The other triangle classification schema approaches triangles from the point of view of the measurements of the internal angles. It, too, designates three types of triangle. 

Acute Triangle: In an acute triangle, the largest internal angle is acute – less than 90°. This means that all the angles are acute. 

Right Triangle: In a right triangle, there is one right angle – an angle of exactly 90°. This means that the other two angles will be acute. 

Obtuse Triangle: In an obtuse triangle, one internal angle is obtuse. This means, again, that the other two angles will be acute.