Defining the Topic

Over at Cliopatra, there's a discussion of the periodization of African history. This periodization stuff doesn't interest me as much as the way we define things geographically. A couple of years ago I wrote some African entries for the 17th and 18th centuries volume of The Greenwood Encyclopedia of Daily Life, and they all involved a discussion of diversity, especially since my purview included the continent's north coast. You can only find so much common ground between Madagascar and Mali. On the other hand, why is it that if I want to do a project that involves both Yemen and Somalia, I'm crossing regional lines?