The four rule

If we add a crease and number the new angles at this vertex,the even and odd angles no longer add up to 180 degrees,and the model doesn’t fold flat.Finally,a layer cannot penetrate a fold.A 2D, flat-foldable base is often an abstract representation of a final 3D shape.Understanding the relationship between crease patterns, 2D bases,and the final 3D form allows origami artiststo design incredibly complex shapes.