A block trade is a high-volume transaction in a security that is privately negotiated and executed outside of the open market for that security. Major broker-dealers often provide "block trading" services—sometimes known as "upstairs trading desks"—to their institutional clients. In the United States and Canada a block trade is usually at least 10,000 shares of a stock or $100,000 of bonds but in practice significantly larger.
For instance, a hedge fund holds a large position in company X and would like to sell it completely. If this were put into the market as a large sell order, the price would sharply drop. By definition, the stake was large enough to affect supply and demand causing a market impact. Instead, the fund may arrange for a block trade with another company through an investment bank, benefiting both parties: the selling fund gets a more attractive purchase price, while the purchasing company can negotiate a discount off the market rates. Unlike large public offerings, for which it often takes months to prepare the necessary documentation, block trades are usually carried out at short notice and closed quickly.
