In the ordinary case: Two facing couples will circle to the left about halfway. The designated man will break his left-hand hold and slide to his left to form one end of a line. The lady with whom he just broke the hand hold with will raise her left hand (still holding the right hand of her partner) and make an arch. Then she will move forward under this arch turning left face under her own left hand to "pull the line straight."
This definition is from The Handbook of Modern Square Dancing by Jay King.
It is provided here for informational and educational purposes only.