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A work is the immaterial concept of a musical piece of art, limited by what's defined in advance ("the score").
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It usually has one or more credits, which are acknowledgments of those who contributed to a work. An artist can be credited as a composer of a work (writing the music), as an author of a work (writing the lyrics), or both.
Example: the work Summertime was written by composer George Gershwin and authors Ira Gershwin and DuBose Heyward.''