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realise or realize, organiser or organizer,
recognised or recognized?

According to the Oxford Dictionary both endings are correct in British English but only one way in American. But make sure you stick to one style within a piece of writing.

The -ize ending is the older, "truer" version and closer to its Greek roots and preferred by some. But the -ise endings are more widely used in British English.

 *CAREFUL a few words must only be spelled with– ise:
advertise, advise, apprise, chastise, comprise, compromise, despise, devise, demise, disguise, excise, exercise, improvise, incise, prise(open), promise, surprise....

 

 British  -ise/-ize  American -ize
 realise / realize
 realize
 apologise / apologize
 apologize
 organise / organize
 organize
 recognise / recognize 
 recognise / recognize 
 finalise / finalize
 finalize
 categorise/categorize  
 categorize