Jishi Koeki refers to the exchange (buying and selling) of rice as a land tax that was collected from koden (fields administered directly by a ruler) by a provincial governor under the Ritsuryo system (a system of centralized government based on the ritsuryo code); rice was exchanged for light commodities that were easier to carry and submit to the central government.
地子交易（じしこうえき）とは、律令制において国司が公田から得た地子稲を中央に上供する際に運搬に便利なように軽貨に交換（売買）すること。