From parallel waves: Those facing in Step Thru to Swing Thru (or Left Swing Thru) and then Turn Thru (or Left Turn Thru). Those facing out turn to face the vacated adjacent spot and promenade single file to the other side and face in as a couple. All now step into a wave. Right hand waves remain so; same with left hand waves.
Other starting formations: From Trade By formation, those facing out, face right and promenade single file; from lines with centers facing in and ends facing out, ends step forward and face in, then pass each other to move to the other side of the set and face in as a pair.
Gus Greene 1971
This definition is from The Handbook of Modern Square Dancing by Jay King.
It is provided here for informational and educational purposes only.