With the beginning of 'Umar's caliphate and the thrust of outward expansion...

With the beginning of 'Umar's caliphate and the thrust of outward expansion, those Arabs who seized the first opportunity to fight, and accordingly migrated to Syria, were organized in relatively cohesive groupings since they belonged to large and homogeneous tribes.

Similarly, in the Basran territories there were mainly two predominant tribes, Tamim and Bakr, and only a negligible number of 300 other people who came from distant areas.[^6] At Kufa, on the other hand, the number of those who came to live from far-off places ranged between 15,000 and 20,000, and were exceedingly heterogeneous in tribal composition. There was a marked absence of large dominating clans or groups of clans.

At first, Sa'd found the solution in dividing them not into individual clans or tribes, but into their broader tribal categories of Nizari (North Arabs) and Yemeni (South Arabs). The Nizaris were therefore quartered on the western side of the plain, and the Yemenis on the eastern side, according to the lots drawn with arrows, as was the custom of the Arabs.[^7] The large plot of land which he demarcated for the mosque was to be the centre of the city.

Adjoining the mosque the governor's residence and the treasury were built. This first arrangement of the population of Kufa, however, had to go through three successive reorganizations in the following 33 years. The organization of the Kufan population into the two broad groupings of the Nizaris and the Yemenis soon proved to be unsatisfactory.

Firstly, neither the various tribes of the Nizaris nor the different groups of the Yemenis found it a congenial to put up together and soon encountered serious problems. Secondly, such an arrangement presented serious difficulties in forming compact military contingents. Kufa was founded as a garrison town intended to furnish well-organized contingents ready for action. This was difficult when people were grouped into two broad divisions.

Finally, the lack of small groupings into clans or groups of allied clans made it difficult to organize the distribution of stipends on which the population depended. Experiencing these difficulties Sa'd, after consulting the Caliph 'Umar, reorganized the population into seven groups.