Reg NMS

Regulation National Market System (or Reg NMS) is a US financial regulation promulgated and described by the United States Securities and Exchange Commission (SEC) as "a series of initiatives designed to modernize and strengthen the National Market System for equity securities." The Reg NMS is intended to assure that investors receive the best price executions for their orders by encouraging competition in the marketplace.

Established in 2005, its aim was to foster both "competition among individual markets and competition among individual orders" in order to promote efficient and fair price formation across securities markets.

In 1972, before the SEC began its pursuit of a national market system, the market for securities was quite fragmented. The same stock sometimes traded at different prices at different trading venues, and the NYSE ticker tape did not report transactions of NYSE-listed stocks that took place on regional exchanges or on other over-the-counter securities markets. This fragmentation made it difficult for traders to comparison shop. In 1975, Congress passed the Securities Acts Amendments of 1975, authorizing the SEC to facilitate a national market system.

In 2005, the rules promoting the national market system were consolidated into REG NMS. Some of the more notable rules include:

The order protection rule has been controversial because it requires traders to transact on a trading venue at the lowest price rather than on a venue offering the quickest execution or the most reliability. Thus, some have described it as an improper government intervention into private business affairs. Defenders of the rule argue that it really just requires what brokers should be doing if they are acting in their customer's best interests. Still others have argued that the rule is too lax because it only protects the quotes at the top of the book. For example, if the best two quotes in one market are superior to the best quote in another market, a portion of an incoming market order may still trade at the inferior market at the inferior price even though the second best quote on the superior market is still available. If more than just the top of the book (the best quote) were protected by the order rule, the market order would have transacted at a superior price and the limit order offering the superior price would have transacted more quickly.

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Wikipedia