Adding endings to words that end in -our
In British English, when you add the endings -ous, -ious, -ary, -ation, -ific, -ize, or -ise to a noun that ends in -our, you need to change the -our to -or. For example:
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humour
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humorous
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glamour
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glamorous
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labour
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laborious
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But when you add other endings, the -our spelling stays the same:
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colour
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colourful
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favour
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favourite
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odour
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odourless
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This rule doesn’t apply to American English: see more information about the differences between British and American spelling.