compendium forms either of two plurals: compendiums or compendia. The native grown plural is slightly preferable--e.g.: "One of the chief shortcomings of compendiums like this is finding what you want quickly" (Seattle Times).
--Garner, Brian A. The Oxford Dictionary of American Usage and Style. New York: Oxford U.,2000.
• DERIVATIVES compendiously adverb.
• ORIGIN Latin compendiosus ‘advantageous, brief’.
--Compact Oxford English Dictionary http://www.askoxford.com/concise_oed/compendious.
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