Of two kinds.
Twofold, double.
A wallet, billfold, or carrying case with a single fold, so that it opens like a book.
A door, window, shutter, or divider consisting of two equal panels hinged together so that it opens by folding the panels against each other.
A sheet of paper or cardboard folded in half along a crease down the center.
A crease or turn that causes something to double back on itself.
that have a similar relationship
A correspondence; the situation where things correspond or match.