Sōryū and Hiryū test a paired-carrier island arrangement

Japan explored paired-carrier operations by placing Hiryū's island to port while Sōryū retained the conventional starboard arrangement. Sailing with the islands adjacent could simplify communication between the ships while leaving the outer flight-deck edges open for aviation. The experiment challenged the safety logic behind the normal starboard island, which kept the structure away from the direction pilots were more likely to veer during a failed approach. Britain also considered coordinated carrier operations but avoided changing the layout in ways that would compromise landing safety. Japan's earlier carriers had consumed its 81,000-ton allowance, restricting construction while designers continued developing their ideas. Sōryū's 1935 launch and Hiryū's 1937 launch fell around Japan's withdrawal from the treaty system, linking this later construction to the removal of cumulative limits.