bundles the purchase of three crude oil futures (30,000 barrels) with the sale a month later of two unleaded gasoline futures (20,000 barrels) and one heating oil future (10,000 barrels). The 3-2-1 ratio approximates the real-world ratio of refinery output—2 barrels of unleaded gasoline and 1 barrel of heating oil from 3 barrels of crude oil. Buyers and sellers concern themselves only with the margin requirements for the crack spread contract. They do not deal with individual margins for the underlying trades. An average 3-2-1 ratio based on sweet crude is not appropriate for all refiners, however, and the OTC market provides contracts that better reflect the situation of individual refineries. Some refineries specialize in heavy crude oils, while others specialize in gasoline. One thing OTC traders can attempt is to aggregate individual refineries so that the trader’s portfolio is close to the exchange ratios. Traders can also devise swaps that are based on the differences between their clients’ situations and the exchange standards. References Oil and gas markets Petroleum economics