Commodity money and commodity markets in a crude early form are believed to have originated in Sumer where small baked clay tokens in the shape of sheep or goats were used in trade. Sealed in clay vessels with a certain number of such tokens, with that number written on the outside, they represented a promise to deliver that number. This made them a form of commodity money - more than an "I.O.U." but less than a guarantee by a nation-state or bank. However, they were also known to contain promises of time and date of delivery - this made them like a modern futures contract. Regardless of the details, it was only possible to verify the number of tokens inside by shaking the vessel or by breaking it, at which point the number or terms written on the outside became subject to doubt. Eventually the tokens disappeared, but the contracts remained on flat tablets. This represented the first system of commodity accounting.