To make a sole fillet easier to remove from the poaching liquid, and to create an attractive package for presentation, it is customarily folded in half (or sometimes rolled up to form something called a paupiette) before being placed in the liquid. When folding a fillet, the smooth outside of the fillet (where the skin was) should be on the inside, and the inside (where the bones were) should be on the outside. If you don’t want to have to remember any of this, just try folding in both directions; the one in which the fish folds flatter is the correct one.