The term 'double over' can refer to three different actions. The first is folding, which is the act of bending a material along a straight line so that one part of it lies on top of another. The second is PLY, which is the act of folding a material in half and then folding it again in the same direction. The third is jackknifing, which is the act of folding a material in half and then folding it again in the opposite direction. All three of these actions involve doubling over a material in some way.