Film sound is carefully designed and plays an important role in storytelling. Sound can be diegetic, originating from visible or implied sources within the film, or non-diegetic, added in post-production from outside the world of the film. Sound is used selectively to emphasize key elements or create atmosphere, and is mixed carefully to maintain the correct atmosphere for each sequence. Film sound can be synchronous to match actions, asynchronous to come from outside the diegetic world, or contrapuntal to create alternative meanings by juxtaposing noise with visuals.