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different as night and day

Also, different as day and night. Totally unlike. For example, Although they are sisters, they are as different as night and day. Also see night and day.

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The American Heritage® Idioms Dictionary Copyright © 2002, 2001, 1995 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company.