Now, on top of their sophisticated numeral system and the writing that they developed, they also had a very sophisticated calendar. Their calendar was more accurate than the Julian calendar, which was adopted by Julius Caesar, and that was the state of the art in the Western world until the middle of the second millennium. So their calendar was more accurate than what was used in most of the world until around 15 or 1600. Now, as we get into 250 in the Common Era, that's considered the Classical Period. That's when some of the really great cities start to emerge, cities like Tikal and Calakmul, and my apologies ahead of time for the pronunciation. It was believed that at their peak, these cities had 50 to 100,000 people. Now, what we now believe is that it was not one unified empire, that there wasn't a Mayan empire with a centralized emperor, but rather it was closer to the Greek city-states, where each of these cities were their own state, their more powerful cities, like Tikal or Calakmul, might have had influence or sway over others, but it wasn't a unified empire in the sense of the Roman Empire or the Persian Empire or some of the early Indian Empires.