Another thing is that there are two very different meanings for billion and trillion - the one that's current in the USA and teh one that used to be current elsewhere. The American usage has now been taken up in Britain as the one normally used, but not always, and not in all countries.

The difference is that the American billion is only 1,000 million, whereas the more traditional meaning is one million million. The other word for the American billion is "milliard", but that seems to have dropped out of usage.