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The points where the lines are known as vertices, while every line segment stands as the sides of the triangle in question; and both sides continued to give rise to the interior angle.

The above shows that the triangle will be formed by three sides as discussed by three vertices, three interior angles and three external angles.

At the depiction on a plane, the triangle will be appointed it with capital letters A, B and C in its vertices and sides may be referred to as AB, BC and AC.

To triangles issue that will yield us three types of triangles can be classified them by two issues, length observed its sides, and on the other hand for the breadth of its angles, classification that will pass four different types of triangles.

Meanwhile, by extension of its sides we will find an equilateral triangle, which is the one whose sides are equal, as they are its interior angles; opposed to this is the scalene triangle, presenting three lengths and in the middle of the two opposite sides we are isosceles triangle, it is characterized by presenting both sides with the same extension.

Note that angles that are opposed to those sides have the same measure. The prominent Greek philosopher such of Mileto showed precisely this question with respect to the isosceles triangle that presents two identical angles, thus imposing a relationship between length and angle.

**Article contributed by the team of collaborators.**