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:
But when you add other endings, the -our spelling stays the same:
This rule doesn’t apply to American English: see more information about the differences between British and American spelling.
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