Although Regroup can be done from any Parallel Lines, it is
almost always called from Back-to-Back Lines. This is
primarily done because of flow considerations for the Outside dancers. EN: 40
Outside 4 always end directly facing each other on the Outside. EN: 50
The Outsides should pass Right-shoulders with each other. Other
glossaries may define the Outsides' part as 1/4 Out &
Cross Cast Back [C1]
(hence implying a Left-shoulder pass). EN: 60