convertible husbandry, and in the hilly and mountainous zones around Mesopotamia (following a kind of transhumance). The relative decline in the cultivation of flax for linen freed land for the growth of cereals as well as sesame, which was introduced to Lower Mesopotamia at this time and was a profitable replacement for flax since it provided sesame oil. Subsequently, this resulted in the development of an important textile industry, attested by many cylinder-seal impressions. This too was largely an institutional development, since wool became an essential element in the maintenance rations provided to workers along with barley. The establishment of this 'wool cycle' alongside the 'barley cycle' (the terms used by Mario Liverani) had the same results for the processing and its redistribution, giving the ancient Mesopotamian economy its two key industries and went along with the economic development of large systems. Moreover, wool could be exported easily (unlike perishable food products), which may have meant that the Mesopotamians had something to exchange with their neighbours who had more in the way of primary materials. Pottery The production of pottery was revolutionised by the invention of the potter's wheel in the course of the 4th millennium, which was developed in two stages: first a slow wheel and then a rapid one. As a result of this it was no longer necessary to shape ceramics with the hands alone and the shaping process was more rapid. Potters' kilns were also improved. Pottery was simply coated with slip to smooth the surface and decoration became less and less complex until there was basically none. Painted pottery was then secondary and the rare examples of decoration are mainly incisions (lozenge patterns or grid lines). Archaeological sites from this period produce large quantities of pottery, showing that a new level of mass-production had been reached, for a larger population—especially in cities in contact with large administrative systems. They were mainly used for holding various kinds of agricultural production (barley, beer, dates, milk, etc.) and were thus pervasive in everyday life. This period marks the appearance of potters who specialised in the production of large quantities of pottery, which resulted in the emergence of specialised districts within communities. Although the quality was low, the diversity of shapes and sizes became more important than previously, with the diversification of the functions served by pottery. Not all the pottery of this period was produced on the potter's