When isn’t it -ize?

When you’re using the -ize spelling rather than -ise, there are some words that retain the -ise ending nevertheless. How can you tell?

The simplest way is to see if there’s an -ization form of the word. If there’s no such word, it keeps the S even when you’re using the -ize spelling (someone will maybe come up with an exception, but it’s a good rule of thumb):

• civilize, realize, santize, organize
  No problem with civilization, realization, sanitization and organization, so these words can be spelled with a Z
• appetiser, circumcise, advise, advertiser
  You can’t have appetization, circumcization, advization or advertization and so these words retain the S
• and various short words like rise, prise, raise and praise, of course

While we’re on the subject, please note that it’s not simply that the Z is American usage, as the Dutch often seem to be taught. It’s fine in British usage too – just choose one and stick to it, and if you get an editor who wants the other simply let them get on with it.

Prevalence: low. The most common issue for -ize/-ise is just that people mix the two up in the same document, which is bad style.
Frequency: medium. Someone who isn’t clear on this will generally make the same mistake multiple times.
Native: rarely. Though American spelling will occasionally include an incorrect Z.