There are many species of sole and flounder, all flat fish with tiny scales and a white belly. The newly hatched fish have one eye symmetrically on each side of the head. When they grow to 2 cm long, one eye begins to move towards the top of the head, then crosses to the other side to lie adjacent to the other eye. As this happens, the pigment on the opposite side becomes lighter and turns almost white.
Both sole and flounder have a gill cover with another bone in front of it. In a sole these are joined together; in a flounder they are separate and you can insert your fingernail between them.
Both varieties of fish have white, soft flesh with a very fine flake, on a fine-boned skeleton. In most recipes, whether they are used whole or as fillets, sole and flounder are interchangable.