The distinction between internationally tradable and non-tradable commodities lies at the heart of the reason for the development of the theory of international trade as an area of economics distinct from the general theory of value. The existence of non-tradable commodities as well as tradable commodities implies that some markets are domestic while others are international. Connections between the prices of these two sets of commodities have been the subject of the famous factor-price equalization and Stopler–Samueslon theorems. Non-tradable goods play a prominent role in the analysis of many problems, such as tariff reform, exchange rates and international transfers.