I've heard it thrown about in different contexts, but I never quite under neither what it does or what dictates when the sound changes to じゅう in a context, as in 世界中. Anyone have any ideas?
中 roughly means "the space inside". In case of 世界中, it means "the space inside the world", which is "everywhere in the world". In case of 絶望中, it means the person is within the time of despair (it probably helps if you imagine time to be long tube or something).