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Reg NMS

The Regulation National Market System (RegNMS) is a comprehensive SEC framework adopted in 2007 that modernized US stock market structure. It mandates that orders be routed to achieve the best available prices, prohibits trades that bypass better prices at other venues (trade-through rule), requires fair access to exchanges, and establishes standards for market data and alternative trading systems. RegNMS is the foundation of today’s multi-venue market structure.

This entry is about market structure regulation. For international equivalents, see MiFID II; for the market infrastructure, see Stock exchange.

The pre-RegNMS market structure

Before RegNMS (pre-2007), the US stock market was dominated by a few large exchanges. Trading fragmentation was limited; the NYSE and NASDAQ handled the vast majority of volume. Information asymmetries were common; not all market participants could see all available prices.

The SEC recognized these inefficiencies and launched a comprehensive market structure reform, culminating in RegNMS.

Key provisions of Reg NMS

Order Protection Rule (Trade-Through Rule). Brokers cannot execute a customer order at a price worse than the best bid or ask available elsewhere in the market. If a customer orders to buy at $100, and the NASDAQ is offering shares at $99.99, the broker must route to NASDAQ, not execute locally at $100.

This rule ensures that retail investors receive fair prices and prevents old, slow venues from executing at stale prices.

Best Execution Rule. Brokers must execute orders to obtain the most favorable overall execution considering price, speed, size, and likelihood of execution. This rule obligates brokers to shop orders among multiple venues and select the best one.

Fair Access Rule. Alternative trading systems must provide fair and non-discriminatory access to market participants who meet reasonable standards.

Market Data Rules. Exchanges must offer standardized, consolidated market data feeds at reasonable costs. This enabled the proliferation of alternative data providers and competition.

Regulation SHO. Provisions on short selling, naked short selling, and short-selling data.

Impact on market structure

RegNMS enabled and encouraged the proliferation of trading venues:

  • Dark pools mushroomed after RegNMS because fair access and trade-through rules meant they could coexist with exchanges.
  • Alternative trading systems expanded in number and size.
  • Lit venues emerged as a distinct category.
  • High-frequency trading was unleashed; RegNMS’s speed-neutral rules allowed profits from speed to exceed profits from information.

The result was far greater trading fragmentation: a single order might be split and routed to a dozen venues to achieve execution.

Trade-through rule: How it works

Suppose an investor places a market buy order for 1,000 shares of Apple:

  • NYSE is showing an ask at $150.00 with 500 shares available.
  • NASDAQ is showing an ask at $149.99 with 1,000 shares available.

The broker must execute at least part of the order at $149.99 (the better price), not at $150.00. If the broker executes at $150.00 without having routed to NASDAQ first (or without finding $149.99), that is a trade-through, potentially a RegNMS violation.

The rule is not absolute; if a broker can demonstrate that routing to the better venue would have caused unacceptable delays or other issues, the trade-through may be excused.

Best execution in practice

Best execution is a broker’s obligation to its customers. A broker examining an order should:

  1. Identify all available venues and prices.
  2. Evaluate the transaction costs (commissions, fees) at each venue.
  3. Consider the speed of execution and likelihood of full execution.
  4. Route to the venue offering the best overall result.

In practice, many brokers use automated order routing that electronically checks multiple venues and routes pieces of orders to the best venues.

Criticisms of Reg NMS

Complexity. The fragmented market is complex; execution costs and strategies have become more sophisticated.

High-frequency trading advantage. Reg NMS’s speed-neutral design allowed high-frequency traders to exploit sub-millisecond price discrepancies, widening spreads for retail investors during volatile periods.

Information asymmetry. Dark pools hide order flow, reducing transparency. Retail investors don’t know what’s trading in dark pools.

Flash crashes. The 2010 flash crash occurred within a RegNMS market structure that, some argue, enabled cascading failures across venues.

International regulation: Differences from MiFID

RegNMS is the US framework. The EU has MiFID II, a similar but distinct market structure framework. Both mandate best execution and fair access but have different specific rules and venues.

See also

Wider context