Regulation NMS

What Is Regulation NMS?

Regulation NMS (National Market System) is a comprehensive set of rules adopted by the SEC in 2005 that governs the structure and operation of US equity markets. The regulation establishes the framework for how exchanges interact, how orders are routed, and how market data is distributed, with the overarching goal of promoting fair competition and protecting investors.

Reg NMS was the most significant reform of US market structure in decades, reshaping the competitive landscape among exchanges and laying the foundation for the modern multi-exchange market system.

The Four Pillars of Reg NMS

The Order Protection Rule (Rule 611) prevents "trade-throughs," where an order is executed at a price inferior to the best available price on another exchange. If Exchange A has the best offer at $50.02 and a trade executes at Exchange B at $50.03, the trade-through has occurred. This rule ensures price priority across the national market system and is the backbone of the NBBO concept.

The Access Rule (Rule 610) addresses the fairness of cross-exchange interaction. It caps the fees exchanges can charge for accessing their quotes at $0.003 per share and requires exchanges to maintain policies for fair access to their quotation systems. This prevents exchanges from making it prohibitively expensive to route orders to their best prices.

The Sub-Penny Rule (Rule 612) prohibits exchanges from accepting or displaying quotes in increments smaller than one cent for stocks priced above $1.00 (and $0.0001 for stocks below $1.00). This prevents sub-penny front-running, where a participant steps ahead of a displayed quote by a fraction of a cent.

Impact on Market Structure

Reg NMS transformed US equity markets. It enabled the proliferation of competing exchanges, growing from a handful to over 15 registered exchanges, each offering different fee structures, order types, and specializations. This competition has tightened spreads and reduced explicit trading costs.

However, the resulting fragmentation has also introduced complexity. Orders may be split across multiple venues, market data must be aggregated from many sources, and the interaction between different exchange fee models creates incentive misalignments. The debate over whether Reg NMS struck the right balance between competition and simplicity continues to evolve.