It's been more than 20 years since the post-rock instrumental band Tortoise released its first album. Now, the beloved Chicago group has a new album called The Catastrophist, which grew out of a commission the band received from the city of Chicago itself to write and perform material that saluted the city's rich history of improvisational music. The purposefully loose structures Tortoise developed for that project took on a somewhat more defined shape as The Catastrophist.

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