Often you hear something like X song was number one on the Top 40 and say, R&B or Modern Rock charts at the same time. Are there any real criteria as to which chart a song enters? Clearly you can tell instinctively if a song is country, but I would think there would nonetheless be true aspects of a song that qualify it. And what about the more vague categories, like Adult Contemporary (what is that, anyway?)?
No ideas? 