I used to think that words ending with the "ise" sound were spelt "ise" in English but "ize" in American English.

I've since read that in English "ize" should be used when adding the "ise" sound to an existing word, eg. summary becomes summarize and final becomes finalize. But "ise" should be used when the "ise" sound is already part of the word, eg. devise, compromise.

Can you please confirm?

(I also realise that both spellings are generally acceptable.)
The original and most widely used is the British ....ise.
The "ise" or "ize" suffix is added to a noun or adjective to form a verb:

standard - (to) standarise or standarize
militar - (to ) militarise or militarize
summary - (to) summarise or summarize

If one spelling is preferred in AmE or BrE, that's another story. Emotion: smile

Now, there are verbs which already go with "ise" and their corresponding noun goes with "ice". Examples:

(to) practise [verb] - practice [noun]
(to) devise [verb] - device [noun]

This distinction is observed mainly in BrE.
Students: Are you brave enough to let our tutors analyse your pronunciation?
Thanks Raul but surely you mean standardise/ize? You ommited the second 'd'. Emotion: smile

And, yes, I agree that both are acceptable but which should be used in BrE? Standardize or standardise? Is there a preference?
Oops! You're right, I typed them too fast and didn't doublechecked. Please, accept my apologies. Emotion: smile

I think the "ise" form is more common in BrE.
 Mike in Japan's reply was promoted to an answer.
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The "original" is -ize. Oxford (the UK university) prefers -ize. -ize is etymologically and phonetically correct.