British spellings should always be employed in the main namespace — also called "namespace 0", or just "main". To give an exceptionally brief overview, this means using -our in a lot of words, such as colour and favour. It also means the general, but not universal, preference for the suffix -ise over -ize. Of course, it goes a bit deeper than that. By most accounts, there are about 1800 differences between US and UK English, with other parts of the English-speaking world chipping in a few hundred other variations.
Redirects
If the title of an article includes a word that is spelled differently in American English than British English, the British form must be used in the title. However, a redirect should be created for the American spelling so as to allow readers to find the article by typing in the American spelling in the search bar. For example, you can find Target novelisation, by entering Target Novelization in the search bar.
Where applicable
This policy is applicable only in namespace 0 — that is, on a page whose name does not have a prefix like Tardis: It also applies where elements from other namespaces may appear in namespace 0. Thus, category names must use British spellings, because category names are visible in namespace 0. Also, the text of templates visible in the main namespace are subject to this policy.
It does not apply to your own user pages, the forums, talk pages, help pages, MediaWiki pages, template documentation, or even project pages like this one. As a rule of thumb, if the text can't be seen on a "normal" page, this policy doesn't apply to it. However, it does apply to the names of templates, because the bot will try to correct those names.
As an example, it applies to World Zones Organisation, the title of {{Infobox Organisation}}, any text that displays on a normal article page as a result of using the template, and the title of Category:Alien defence organisations.
But it does not apply to the text at category:Alien defence organisations, nor to the text at template:Infobox Organisation, nor in a forum thread called Forum:Organization or Organisation?, nor in template documentation.
Exceptions
- American topics should get American spellings. Don't try to Anglicise "Pearl Harbor", "World Trade Center" or the film, The Color of Money (unless those spellings are represented in actual DWU sources — but in case both variants exist, the real-world-accurate American one should still be given preference for the page name).
- Our rulings about specific words aren't gospel. A good example of this is Similarities in Proto-Cultural Artifacts of the Second Dynasty of the Zyrs. Despite the fact that we have, as a community, chosen to spell the word artefact, the spelling artifact must stand in the case of this particular fictional book title.
- In the case of products aimed at a non-British market, their spellings and word choices should predominate. IDW Publishing titles should always reflect American spellings, as should anything to do with Doctor Who Insider magazine, or BBC America.
Related reading
You'll want to read T:QM and T:HONOR for advice about other trans-Atlantic issues.