# What is the principle of folding chairs?

In the design of folding articles, the stability of the triangle and the instability of the quadrilateral are mostly used. So what is the stability of the triangle? If you remove an edge from a triangle, the triangle becomes a "corner" that can rotate freely, but if the three sides of the triangle remain the same, then the shape and size of the triangle will not change anyway, the triangle The nature is called the stability of the triangle.

What is the instability of the quadrilateral? The quadrilateral does not have stability, that is, the shape of the quadrilateral can be changed after the length of the four sides is determined. These two properties have a wide range of applications in daily life and production, such as bicycle girders, car roof sheds, flower market scaffolding, billboards, store name brackets, crane jibs, etc. They are all triangular structures, and the principle applies the stability of the triangle. Another example is the electric sliding door of the school, and the protection door of the store uses the instability of the quadrilateral.

Since things can be transformed into each other under certain conditions, if the quadrilateral is changed into a triangle, then the instability of the quadrilateral can be transformed into stability. Conversely, if the triangle is changed into a quadrilateral, then the stability of the triangle can be transformed. It is the instability of the quadrilateral. The design of the folding article utilizes the principle of mutual transformation of things. The quadrilateral instability in the folding process can be folded at will, and the stability of the triangle is used when using the function.