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Grove Music Online (grovemusic.com)
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March, Ivan, Edward Greenfield, and Robert Layton. The Penguin Guide to Compact Discs and Cassettes. London: Penguin Books Ltd, 1994, 2001.
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Simms, Bryan R. Music of the Twentieth Century. New York: Schirmer Books, 1986.
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Wikipedia (en.wikipedia.org)
The medieval section condenses about 550 years into one column, but from 1400 on spacing is proportional (1/16″ = 1 year). Thus the rectangles enclosing first- and second-rank composers exactly correspond to their life spans and indicate precisely who lived when. Third- and fourth-rank composers are arranged by year of birth and (mostly) alphabetically from top to bottom. They’re also staggered to give a rough indication of their life spans.