A diagonally braced garden gate

Collapsing Quadrilaterals, Rigid Triangles

All four bars are in place, every screw tight. A year later the gate has leaned over: the latch no longer meets its catch, the bottom corner drags in the dirt. No bar got longer, no screw came loose. So four bars of fixed length can change shape without any of them changing. Something is missing from a quadrilateral — and the missing thing is not an angle, it is a length.