Chain Down the Line starts from a right-handed two-faced line or a left-handed wave, most commonly with girls in the center. The center dancers trade (turning 1/2 by the right) and then do a Courtesy Turn with the dancer from the other end of the line/wave.
Each end dancer will form a couple with the center dancer who was originally in the other half of the line/wave, and the two couples will be facing each other.