Regulation NMS
The rules that protect investors by ensuring best execution across exchanges.
Regulation NMS (National Market System) is a set of SEC rules adopted in 2005 that governs how securities trade across multiple exchanges in the United States. Its core purpose is to ensure that investors receive the best available prices when their orders are executed. The most important component is the Order Protection Rule, which prevents exchanges from executing trades at prices worse than the NBBO displayed on competing exchanges.
Why It Matters
Reg NMS is the reason you can trade with confidence that you are getting a fair price, regardless of which exchange your broker routes your order to. Before Reg NMS, it was possible for your order to be filled at a worse price on one exchange while a better price was available on another. The regulation eliminated this "trade-through" problem.
For options traders, Reg NMS principles extend through related rules that govern options markets. While Reg NMS technically applies to equities (NMS stocks), options markets have analogous protections through exchange rules and the Options Linkage Plan, which serve the same purpose: ensuring you get the best available price across all options exchanges.
How It Works
The four pillars of Reg NMS:
Order Protection Rule (Rule 611): Prohibits trade-throughs — executing an order at a price worse than the best displayed price on another exchange. If the NYSE bid is $50.10 and Nasdaq's bid is $50.05, a sell order must be routed to NYSE to get the better price.
Access Rule (Rule 610): Ensures fair access to quotations by capping the fees exchanges can charge for accessing their quotes. This prevents exchanges from posting attractive quotes but charging excessive fees to access them.
Sub-Penny Rule (Rule 612): Prohibits quoting most securities in increments of less than one cent. For options, this translates into minimum tick sizes (penny or nickel increments). The Penny Pilot Program allows certain liquid options to trade in penny increments.
Market Data Rules (Rules 601 and 603): Require the fair distribution of market data and revenue sharing among exchanges that contribute to the consolidated data feeds.
How Reg NMS affects your trading:
- Your limit orders are protected from being traded through at an inferior price
- Fees to access quotes are capped, keeping execution costs reasonable
- Market data is available from all exchanges, enabling accurate NBBO calculations
- Competition among exchanges is encouraged, generally resulting in tighter spreads
Criticisms and unintended consequences:
- Reg NMS led to market fragmentation (16+ options exchanges today), which creates complexity
- High-frequency traders can sometimes exploit tiny time differences between exchange data feeds
- The regulation may have inadvertently encouraged the growth of dark pools as venues to avoid the rules
- Some argue the rules have not kept pace with modern market structure and need updating
Quick Example
You sell 10 contracts of a $100 put on the CBOE. The best bid on CBOE is $3.20, but the best bid on PHLX (another exchange) is $3.25. Under Reg NMS principles (applied through the Options Linkage Plan), your order should be routed to PHLX or you should receive at least $3.25.
Without these protections, CBOE might fill you at $3.20, and you would miss the $0.05 per share better price — costing you $50 across 10 contracts. Reg NMS ensures this trade-through does not happen, protecting your execution quality across the fragmented exchange landscape.