From lines facing out: Pair the Line In -- Centers "backtrack" (U-turn back by turning in toward each other). Ends Fold behind the centers. Finishes in double pass thru formation. From lines facing in: Pair the Line Out -- Centers backtrack to face out and ends Fold in behind them. Finishes in completed double pass thru formation.
NOTE: Also called Pair It In/Out; Lines Pair In/Out.
George Elliott 1958
This definition is from The Handbook of Modern Square Dancing by Jay King.
It is provided here for informational and educational purposes only.