It used to be that one company was listed on one exchange and that exchange was the most common venue for trading in that company shares. This is referred to as a public market for trading shares, and the liquidity of a stock is displayed to all market participants since the order book for each company is visible to all. This is sometimes referred to as lit liquidity. Increasingly, more and more trades are done on alternative trading systems. Most known of these alternative trading systems are dark pools which gained popularity through the early 2000s. Dark pools are referred to as private markets and have the advantage of not showing market participants, the volume of orders for any company which have yet to be matched. In other words, the order book is not visible for all to see, and liquidity is therefore not displayed. Dark pools are more attractive for institutions wishing to place large orders which run the risk of revealing the intentions of the investor to the rest of the market. If the order is not immediately matched and appears on a visible order book. Liquidity is therefore not displayed or in other words, dark pools have dark liquidity. Many investment banks run their own dark pools. It's also worth noting that alternative trading venues are under increased scrutiny from regulators especially following a few scandals in the space with some banks allegedly favoring certain investors over others, notably high frequency traders in their dark pool. Alternative trading systems can also go by other names such as electronics communications networks, and crossing networks. But all of these broadly offer a similar solution to dark pools.