The Classic Mayan civilization established itself between 200 BC to 900 AD, during which major city-states like Tikal, Copan, and Chichen Itza rose and became urban centers. Agriculture of crops like maize and trade of goods such as quetzal feathers and jade supported these cities and led to the development of social classes headed by a king. The Mayans also had a sophisticated written language consisting of glyphs and independently developed the concept of zero.