A folding camp chair is made up of a canvas seat and aluminum legs joined at their midpoints as shown. Why will this design insure that the seat, when set up, will always be parallel to the ground?

Question

Answer 1

The crossed legs form diagonals of a quadrilateral and since they intersect at their midpoints, the quadrilaterals formed must be parallelograms. Thus the seat and the ground, as “opposite sides,” will be parallel.