“Focussed” or “focused”? The double consonant
The rule that I learned was that when you have a short vowel in the last syllable, you double the last consonant before adding the ending. Thus "cut" becomes "cutting", or "beget" becomes "begetting".
According to this rule, "focus" should become "focussing". But the spell checker in my browser wants me to change that to "focusing". This bothers me because I would think that would be pronounced something more like fo-kyoo-sing.
I looked this up, and apparently the Brits follow the rule, but Americans don't. Any idea why? And what is the rule, then?