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)
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)
Jerry Salisbury 1969
This definition is from The Handbook of Modern Square Dancing by Jay King.
It is provided here for informational and educational purposes only.