Some stock markets rely on professional traders to maintain continuous bids and offers since a motivated buyer or seller may not find each other at any given moment. These are known as specialists or market makers.
A two-sided market consists of the bid and the offer, and the spread is the difference in price between the bid and the offer. The more narrow the price spread and the larger size of the bids and offers (the amount of shares on each side), the greater the liquidity of the stock. Moreover, if there are many buyers and sellers at sequentially higher and lower prices, the market is said to have good depth.
Matching buyers and sellers of stocks on an exchange was initially done manually, but it is now increasingly carried out through computerized trading systems. The manual method of trading was based on a system known as the open outcry system, where traders used verbal and hand signal communications to buy and sell large blocks of stocks in the trading pit or the exchange floor.
However, the open outcry system has been superseded by electronic trading systems at most exchanges. These systems can match buyers and sellers far more efficiently and rapidly than humans can, resulting in significant benefits such as lower trading costs and faster trade execution.