Triangles have been used extensively throughout history in construction due to their rigid shape and strength. Unlike a square, that can shift into a parallelogram when force is applied to one of its sides, an equilateral triangle's sides and angles are fixed.
Due to the rigidness of the triangle's shapes, high amounts of forces can be sustained without deformation. Triangles are often used as trusses in bridges, or other support structures in buildings. They can easily be fit together to provide strength and stability over a wide area. The most famous triangles in structures are the pyramids of Egypt, which are the largest pyramids in the world.