The bid/offer spread (also known as bid/ask spread) for assets (such as stock, futures contracts, options, or currency pairs) is the difference between the price available for an immediate sale (bid) and an immediate purchase (ask). The trader initiating the transaction is said to demand liquidity, and the other party (counterparty) to the transaction supplies liquidity. Liquidity demanders place market orders and liquidity suppliers place limit orders. For a round trip (a purchase and sale together) the liquidity demander pays the spread and the liquidity supplier earns the spread. All limit orders outstanding at a given time (i.e., limit orders that have not been executed) are together called the Limit Order Book. In some markets such as NASDAQ, dealers supply liquidity. However, on most exchanges, such as the Australian Securities Exchange, there are no designated liquidity suppliers, and liquidity is supplied by other traders. On these exchanges, and even on NASDAQ, institutions and individuals can supply liquidity by placing limit orders.
On United States stock exchanges, the minimum spread for many shares was 12.5 cents (one-eighth of a dollar) until 2001, when the exchanges converted from fractional to decimal pricing, enabling spreads as small as one cent. The change was mandated by the U.S. Securities and Exchange Commission in order to provide a fairer market for the individual investor.