Formerly, a clearing member of both the Mid-America Commodity Exchange (MidAm) and another futures exchange who, for a fee, would assume the opposite side of a transaction on MidAm by taking a spread position between MidAm and the other futures exchange that traded an identical, but larger, contract. Through this service, the changer provided liquidity for MidAm and an economical mechanism for arbitrage between the two markets. MidAm was a subsidiary of the Chicago Board of Trade (CBOT). MidAm was closed by the CBOT in 2003 after all MidAm contracts were delisted on MidAm and relisted on the CBOT as Mini contracts. The CBOT still uses changers for former MidAm contracts that are traded on an open outcry platform.
Source: Commodity Futures Trading Commission