What's the difference between Whisky and Whiskey?

It has been generally accepted that whisky (without the 'e') is used in reference to Scotch, Canadian and Japanese whiskies. With the addition of the 'e', whiskey is used in reference to Irish and American whiskeys. There are always exceptions however such as the American whiskies of Maker's Mark and George Dickel who both use whisky. It is really the decision of the producer as to whether to include the 'e' or not and has more to do with marketing than law.