With the great development of trading in the period of Babylon, a seal was used not only for clarifying ownership of possessions but for confirming a contract of trading so letters appeared on its impression. Legal effect of sale contract of all documents involved with the business was guaranteed only by stamping impression of person concerned in the trading on it.
For example, when dealing a commodity, a secretary specialized in commercial transactions writes contractual coverage such as payment condition in sphenogram on a wet clay table with a pen made by stalk. After that, to confirm a contract, an impression of a cylinder seal which a dealer possesses is rolled on a wet clay tablet. When soil gets dry and becomes hard, fulfillment of contract is guaranteed by this unassailable evidence literally.
Moreover, in order to prevent a contractual coverage from being changed later, the following countermeasure against this injustice was taken.
After writing a contractual coverage on a clay tablet, another clay is brought to be stretched into as thin layer as skin to cover a contract clay tablet. After that,
secretary writes the same contractual coverage on this additional external clay and dealers stamp their seal on it again.
If a doubt of forgery arises in a contract document later, a judge breaks the external clay layer and pick up a contract clay tablet and check the contract details written on it and then authenticity of a contract is established.