"Breakthru" was developed in the 1960s as part of the 3M bookshelf game series. It features tafl-like symmetry, but with twelve defenders plus one "flagship" (cf. "king") pitted against twenty attackers upon a tiered board, so that the objective of the defenders is to escort the flagship from the centre to the outer zone of the board. Apart from the distinction of the inner zone and outer zone, there are no distinctive spaces on the Breakthru board. Breakthru also features a distinctive double move, whereas no evidence points to such a move in any of the historical games.