A graph superimposes the growth–decline curves of major Stirrup Rider Empires, from 600 to 1200. While being a major advance in horse riding, the stirrup just offers a short term for the intermediary phase of Rider Empires. Expansion of Islamic Caliphate was the towering event. It surpassed the Xiongnu area record. Apart from Tang, Tufan in Tibet, Liao, and Seljuk, all other medieval empires remained of modest size. In population, Song in 1125 briefly reached 38% of the world population – the largest percentage any empire has ever reached. The Caliphate altered the language mix throughout North Africa, introducing the Arabic. The Seljuks did so from Central Asia to Anatolia, introducing Turkic. The Caliphate clashed with Tang in Central Asia in 751. The forces of an empire reaching to the Atlantic Ocean confronted for the first time those of an empire that reached to the Pacific. Neither realized the momentousness of this skirmish. Western Europe developed feudalism, a maddeningly complex multistranded hierarchical order, which does away with single territorial authority.