The Sumerians had a complex assortment of incompatible number systems, and each city had its own local way of writing numerals. For instance, at about 3100 BC in the city of Uruk, there were more than a dozen different numeric systems.[11] In this city, there were separate number systems for counting discrete objects (such as animals, tools, and containers), cheese and grain products, volumes of grain (including fractions), beer ingredients, weights, land areas, and time and calendar units. Furthermore, these systems changed over time; for instance, numbers for counting volumes of grain changed when the size of the baskets changed.