The Concert only appears in the episode "Choir Boys."
It looks like it is built inside a giant clam and the front part has the bow of a boat with windows and doors. There is also a wooden sign that says "Concert" on it.
Towards the end of the episode of Choir Boys, the Bikini Bottom Men's Choir and SpongeBob performed here for a concert, while Squidward got to turn the pages of SpongeBob's songbook.
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