Law of one price
The law of one price (LOOP) states that in the absence of trade frictions (such as transport costs and tariffs), and under conditions of free competition and price flexibility (where no individual sellers or buyers have power to manipulate prices and prices can freely adjust), identical goods sold in different locations must sell for the same price when prices are expressed in a common currency. This law is derived from the assumption of the inevitable elimination of all arbitrage. Overview. The intuition behind the law of one price is based on the assumption that differences between  prices are eliminated by market participants taking advantage of arbitrage opportunities. Example in regular trade. Assume different prices for a single identical good in two locations, no transport costs, and no economic barriers between the two locations. Arbitrage by both buyers and sellers can then operate: buyers from the expensive area can buy in the cheap area, and sellers in the cheap area can sell in the expensive area. Both scenarios result in a single, equal price per homogeneous good in all locations. "For further discussion, see Rational pricing". Example in formal financial markets. Commodities can be traded on financial  markets, where there will be a single offer price (asking price), and bid price. Although there is a small spread between these two values the law of one price applies (to each). No trader will sell the commodity at a lower price than the market maker's bid-level or buy at a higher price than the market maker's offer-level. In either case moving away from the prevailing price would either leave no takers, or be charity. In the derivatives market the law applies to financial instruments which appear different, but which resolve to the same set of  cash flows; see Rational pricing. Thus: A similar argument can be used by considering arrow securities as alluded to by Arrow and Debreu (1944). The law of one price has been applied towards the analysis of many public events, such as: