A trading practice in stock exchanges in which the same security is simultaneously bought and sold, through a single stockbroker, for the same amount of securities and at the same exchange rate. In Spain, this practice is regulated by Royal Decree 1416/1991 on special stock exchange transactions, off-exchange transactions, and weighted average quotations, and Ministerial Order of 5 December 1991 on special stock exchange transactions. The trading system used for matching transactions is an open outcry trading session. Meanwhile, back-to-back transactions can only match if there are no other counterparts at the same price or more favorable securities at that time. Current regulations allow for transaction matching to take place outside of the normal trading hours and trading system, provided that certain price and volume requirements are fulfilled and the transaction is reported immediately to the market supervisory authorities.