So we've got a common tongue, more or less, but spelling is a different matter. What are the pesky words that are spelled differently in US English and British English? Not the words that fall into an accepted pattern, i.e., or/our, ize/ise, er/re, æ/e, ence/ense, ll/l, g/gue and so forth, but the quirks.
I came up with about 15 off the top of my head, but there must be many more. Current usage only and the decision of the judge will be final.
