The National Market System (NMS) is a regulatory mechanism that governs the operations of securities trading in the United States.
[4] The SEC was also directed to work with self-regulatory organizations in planning, developing, operating, or regulating a national market system.
[5][2] The primary self-regulatory organization regulating member brokerage firms and exchange markets until 2007 was the National Association of Securities Dealers (NASD).
In 2007 the NASD and NYSE member regulation group combined into the Financial Industry Regulatory Authority (FINRA).
The National Market System includes and regulates all the facilities and entities which are used by broker-dealers to fulfill trade orders for securities.