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In page Tempo:

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While many composers have retained traditional tempo markings, sometimes requiring greater precision than in any preceding period, others have begun to question basic assumptions of the classical tradition like the idea of a consistent, unified, repeatable tempo. Graphic scores show tempo and rhythm in a variety of ways. Polytemporal compositions deliberately utilise performers playing at marginally different speeds. John Cage's compositions approach tempo in diverse ways. For instance, 4′33″ has a defined duration, but no actual notes, while As Slow as Possible has defined proportions but no defined duration, with one performance intended to last 639 years.[citation needed]