Starting at a basic level, TRIANGLES are the strongest, most stable 2-dimensional shapes or frames that can possibly exist, in the universe as we know it. This can be shown quickly and easily by a simple process of elimination: TWO sides is NOT enough to form an enclosed frame, and FOUR OR MORE sides become inherently unstable.
The geometry of relationships
In contrast to the above, TRIANGLES can withstand forces from any direction, and the pieces that make them up will shift into either compression, or tension, as necessary. The next page shows examples of multiple different types of structures, and ALL of them are assembled by combining triangles together, into larger shapes. There is no other shape that can be used as this type of "building block" when building strong and durable structures.