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:

• humour / humourous
• glamour / glamorous
• labour / laborious

But when you add other endings, the -our spelling stays the same.

For example:

• colour / colourful
• favour / favourite
• odour / odourless

Adapted from: Oxford Dictionaries: Spelling Rules and Tips (http://www.oxforddictionaries.com/words/spelling-rules-and-tips)