Why the shape "TRIANGLE" is widely used in structural engineering?

Most of the structures that we know comprise of this particular shape "triangle" in any one of its structural systems. 

What is so important? or why "TRIANGLES"?

Just think of basic 2-dimensional geometrical shapes, what do we have? 

Squares, Rectangles, Triangles, circles, pentagon, hexagon, and the list go on.

Of these shapes, why a triangle is called the most stable shape. Let us compare these shapes and understand them.

For our example, let's consider the square and the triangle as shown below.

Applying a force on one of the edges and studying the behavior would lead to a better understanding regarding the stability of the structure.

First, let’s try with the vertical force.

What will happen? The load applied over the square moves directly downward through the vertical member and stands still. The triangle when subjected to a vertical force through one of its edges, distributes that force evenly to either side and stands stable.

Now, let’s apply the horizontal force.

The square could not withstand this force without deformation, it tries to sway along the direction of application of the force and distort from its original shape. Whereas, applying the horizontal force tries to push one edge of the triangle.

It is not possible to move the edge without the elongation of one of its sides, this makes them stay stable for the applied force.

This simple comparison applies to all the other shapes too. Also, this can be easily verified by making a simple model with popsicles. I tried it myself, and recommend you to do so.

To read the extended version that emphasizes the application of triangles in the structural system, go through the link below.