When night falls, we find peace in the knowledge that daytime will return soon, and vice versa. But what if the world was split: daytime and night are two different, coexistent worlds, each with it's own laws and rules. At dawn and sunset, our soul switches to inhabit our other being, our 'twin'.
An army and navy who work in the same way as others except their rank titles are upside-down. So people start as full generals and those with the rank of private command armies.