Sekai (セカイ, World) are mysterious worlds different from the real world.
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There are different Sekai worlds, each one answering the wishes the of creators who want to turn their feelings into songs. The feelings of a person are put in physical form inside the Sekai. Virtual Singers appear in these worlds, with different appearances in each Sekai. They help people discover their true feelings.
People can travel to Sekai using "Untitled" songs. These songs have no melody or song and are created at the same time as the Sekai. Playing an "Untitled" song will transport the listener to the Sekai.