12. -ize or -ise?

A simple way of explaining the difference between the -ize and -ise forms in the spelling of verbs like nationalise/-ize and their derived nominal forms would be to note that -ize is American and -ise British English. But that would be too simple. In recent years so many words have come into British English from American English – energize, optimize, systematize to name just a few – that the -ize form has come to be accepted by many people as the basic form. For instance, if you look up the above examples in an up-to-date dictionary, either English-Dutch or English-English, then you will find -ize as the first spelling variant and -ise as the second. Fifty years ago this would have been the other way around.
     Although there are still very many British people who use the -ise form (just read a few different newspapers or read the news online), we advise you to follow the modern tendency, supported by the lexicographical authorities, and use the -ize form as the basic form. You should definitely do so if you are writing a text that is specifically for an American readership. Finally, if you are bent on using -ise, then make sure you are consistent.
     The story would also not be complete without the exceptions. There is one major group, consisting of words where -is- belongs to the stem rather than being part of a suffix. These verbs are written with -ise in both British English and American English:

There is also a small group of verbs ending in -yse, notably analyse, catalyse and paralyse, which are spelled with the -yse suffix in British English but with -yze in American.