- The -ize spelling is exclusively US = MYTH
- For words with an -ise/-ize or -yse/-yze alternation, the -ize spelling is used in British and World English as well as in US English.
- (The same applies to derivatives, e.g. organisation/organization, organisable/organizable, etc.)
- The -ize spelling is far from being a modern invention.
- Some authoritative British journalism style guides recommend the -ise spelling.
- Overall, there is a marked preference in British English writing for the -ise, -yse spellings.
- While many words can be spelled/spelt either way, a small group always end in -ise (see later in the blog).
- Words spelled/spelt -yse/yze, e.g. analyse, are best written exclusively as -yse in British English.
An evergreen myth
The BBC’s Today programme on Radio 4 is the premier UK radio news programme, with episodes lasting three hours Monday to Friday, and two hours on Saturdays.
In their last ten-minute slot before signing off, they often have a light-hearted linguisitc snippet. So it was that on 28 November there was discussion about the alleged decline in children’s spelling. As if to disprove that trend, we had a ten-year-old official “child genius” who could rattle off the spelling of obscure polysyllables such as eleemosynary.
At some point, the question arose of whether another sesquipedalian word, lyophilisation, should be spelt -isation or -ization. There seemed to be a consensus among guests and presenters that the spelling with -s- was the “English” (Ahem. Read “British”) spelling and the second “American”.
Many British people also believe that there is a hard-and-fast rule: in American English you spell such words -ize, and in British English you spell them -ise.
For the dozens of common verbs which can be spelled/spelt either way, e.g.
glamo(u)rize / glamourise
romanticize / romanticise
socialize / socialise
trivialize / trivialise,
it is true that the -z spelling is standard in US usage. 
However, in Britain, too, it is perfectly acceptable to use the -ize spelling, though the -ise spelling is more widely used . The only problem is that British people who are not editors may well turn up their noses at the-ize spelling, and assume you are a) trying to be unpatriotically transatlantic, or, worse still, b) a Trumpnik. It will also depend on whether you are writing for an organization that has a particular house style, and who the eventual readers are.
Who sez which to use?
Different authorities and institutions have different views. Oxford University Press, for example, favours the -ize spelling, but Cambridge University Press prefers -ise, as do The Guardian, The Economist and The Telegraph. Choosing one form or the other is part of their “house style”: the rules they lay down for their writers.
While you may think it doesn’t matter — and, indeed, in the grand scheme of things (whatever that is), it matters not a jot — it does matter to editors and to journal publishers because they have to make a decision about which style to plump for, and then apply it consistently.
For example, a major academic journal publisher has this in its UK style bible for editors:
“Where UK authors have used -ise spellings throughout their papers in a consistent fashion, please do not change. Where there is inconsistency, use –ize.”
The last sentence of the advice thus shows that, even for the UK, this publisher prioritiz/ses the -z- spelling.
If you are not bound by a house style, you can make up your own mind whether to use -ise or -ize. It’s a matter of personal preference, like Lapsang Souchong vs Green tea.
The important thing is to be consistent within a document, or series of documents, for a given client.
But do bear in mind that if you are writing for the British market, some readers may scratch their heads when they see -ize spellings, so that could distract them from your message. On the other hand, many Americans will simply consider the –ise spelling wrong.
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So, which words must I always spell -ise, no matter whether I’m British, American, etc?
Here are some very common, and a few rather less common, ones:
They are spelt/spelled like that for several reasons, but often because the -ise part came into English from French words that had never had the Greek/Latin -ize spelling. 
One that is a bit of an odd person out and potentially confusion is prise/prize, in the meaning of “Use force in order to move, move apart, or open (something):I tried to prise Joe’s fingers away from the stick.” Even though its origin is French prise, in US dictionaries it has a z, which means it would be a homonym of prize = to value. However, as the comment below by Laura D suggest, nowadays people don’t write it that way, and dictionaries need to update.
 For example, if you look up organize in the ordinary Merriam-Webster online dictionary, the -ise spelling is not acknowledged at all; it is only when you look at the medical dictionary that you see it.
 As noted under various entries in The Cambridge Guide to English Usage.
 For example, advertise came directly from Anglo-Norman and Middle French a(d)vertiss-. Even so, the OED notes: “From an early date the ending was frequently either apprehended [i.e. “interpreted and understood”] as or assimilated to -ize suffix”. Televise, in contrast, is a back-formation from television, and thus the s faithfully respects the word’s etymology.