That last statement is not completely true. The so-called Oxford spelling uses the z, which you can read about here.

The spelling with a z agrees with the original root -izo in Greek words. Other English words come from Greek words with an s in their root. Such English words therefore have an s. This applies to words spelled with a y, like analyse, catalyse, dialyse and paralyse. However, you will find many instances of the spelling -yze in American English.

Some verbs must be spelled -ise in both American and British English. Again, even if we state that -ise is the correct spelling of these words, Americans use -ize in some of them.

You should always spell the following verbs with -ise:

advertise
advise
apprise
comprise
compromise
despise
devise
disguise
excise
exercise
improvise
promise
revise
supervise
surmise
surprise
televise

You can read about words ending in -ice or -ise here.

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