Regulation NMS

Before 2005, US stock exchanges were more like separate shops than one connected market. Nothing legally forced them to honor each other's prices — an exchange could execute your order at a worse price than another exchange was quoting at that exact moment, and you'd have no automatic protection. Regulation NMS ("National Market System") is the SEC rule that stitched every US stock exchange into a single, price-linked whole, and it's the piece of regulation underneath almost everything about how US equities trade today.

The four parts

Reg NMS is really a bundle of four separate rules, each solving a different problem:

Rule	What it does
Order Protection Rule	Bans "trade-throughs" — executing at a worse price when a better, immediately accessible price is displayed elsewhere. See The Order Protection Rule And Trade-Throughs.
Access Rule	Requires fair, non-discriminatory access to quotes across venues, and caps the fee a venue can charge for taking liquidity at its best price. See The Access Rule And Fee Caps.
Sub-Penny Rule	Bans quoting most stocks in increments smaller than a penny, to stop venues from stepping ahead of each other by a fraction of a cent.
Market Data Rules	Governs how exchanges must consolidate and distribute a single best-bid-and-offer feed across all venues — the The NBBO And The Consolidated Tape.

Why this mattered: a single example

Say Exchange A is quoting a stock at 50.00 bid, and Exchange B is quoting the same stock at 50.02 bid, both displayed and both immediately accessible. Before Reg NMS, a broker routing a sell order to Exchange A could simply execute it there at 50.00, even though a client selling that stock deserved the better 50.02 available two feet away, electronically speaking, at Exchange B. The Order Protection Rule makes that illegal: a venue may not execute a trade that "trades through" a better price displayed on another venue, and must instead route the order to reach that better price, or match it.

What it changed about market structure

Reg NMS is also, indirectly, why the US has a dozen-plus stock exchanges instead of one or two. Because every venue is legally required to honor the best price no matter where it's quoted, a new exchange doesn't need overwhelming market share to matter — even a small venue's best quote must be respected by everyone else, which lowered the barrier to launching new venues and is part of why the exchange landscape fragmented rather than consolidated after 2005. It's also the regulatory backbone behind maker-taker pricing: because the Access Rule caps what a venue can charge to take liquidity, venues compete instead by rebating the maker side, which shaped order routing incentives across the whole industry.

Reg NMS is a US-specific framework; Europe's roughly equivalent regime is built around MiFID II And Best Execution, which imposes a similar "get the client the best outcome" obligation but through a broader, more principles-based best-execution duty rather than a hard trade-through ban tied to displayed quotes.

What it did not solve

Reg NMS ties venues together on price, but it says nothing about speed. Because every venue's quote must be respected, but different venues sit at different physical distances from any given trader's servers, the rule inadvertently created a race: whoever can see and react to a quote change fastest gets to trade on it before anyone else, even though every venue is nominally on equal footing under the rule. This is a large part of why colocation — paying an exchange to place your own servers in the same data center as its matching engine — became commercially important after 2005, and why some venues later introduced deliberate speed bumps specifically to blunt that advantage rather than reward it.

It's also worth being precise about what Reg NMS requires versus what it merely permits. The Order Protection Rule requires venues to avoid trading through a better displayed price, but it does not require every venue to quote competitively, and it does not ban internalization or off-exchange trading outright — a broker can still execute a client order away from the public market entirely, provided the price given is at least as good as the protected NBBO. That carve-out is the regulatory foundation underneath both wholesaler internalization and Payment For Order Flow, and it's a frequent source of confusion: Reg NMS guarantees a floor on execution quality relative to the displayed market, not that every order actually reaches a lit exchange.