The Medieval Mongol ulus was a category of government that was turned into a 'community of the realm' and as such it was assumed to be 'a natural, inherited community of tradition, custom, law and descent', a 'people' or irgen. According to Mongolian language sources of the thirteenth and the fourteenth centuries, 'Mongol' was the only contemporary ulus and irgen while all the other contemporary categories such as Kitat, Tang'ut and so on were described only as irgen. Analysing this usage convention, this paper reconstructs medieval Mongol concepts of ulus, irgen and yeke ulus. The idea of ulus congruent with the Chinggisid state of 1206 was well established in pre-Chinggisid Mongolia. Indeed, the Mongol ulus of 1206 was a realisation and an embodiment of that idea, and was built upon the Kereyid state and her sphere of hegemony. As such, the Mongol ulus of 1206 was different and distinct from the Yeke Mongqol Ulus, the Mongol Empire. This finding not only renders the idea of the tribal origin of the Chinggisid state untenable, but also suggests that we must look at the legacies of previous state formations for the origin of the Chinggisid state.