B stage
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In some pop and rock concerts held in arenas and stadiums, a B stage is a small secondary stage, usually located in the middle of the concert floor, connected to the main stage by a catwalk of some sort.
The B stage began being used in the early 1990s by groups such as U2 and The Rolling Stones. It provided a more intimate setting for stripped-down, quieter versions of songs, that could be played in greater proximity to the audience.