I'm wondering about certain words that can be spelt with either a S or a Z, for example, recognise/recognize, customise/customize etc. I spell them with an S and I had previously thought that it was a British thing and that it was Americans who used the Z spelling, but then recently I've noticed a lot of British books using the Z spelling but still the British forms of words like 'colour' and 'rumour'. I've never seen an American text use the S spelling however.
So, are both the S and Z versions acceptable in both countries? Are there any rules regarding their use?