Version #1

From one pair behind another or pairs back to back: Designated couple does a U-turn back and steps in between the other couple to form a line. (author unknown)

Version #2

From various formations: Turn In--those having their backs to the center of the set will U-turn back to face in. Turn Out--those facing the center of the set will U-turn back to face out. (Jerry Salisbury 1969)