Intermarket Trading System Overview

The Intermarket Trading System (ITS) was a linkage among US stock exchanges that operated from 1978 to 2006, designed to prevent trade-throughs—the execution of orders at worse prices when better quotes existed elsewhere. Though it eventually gave way to Regulation NMS, ITS represented the first systematic effort to make equity markets behave as a national whole.

How the Intermarket Trading System Was Built

Before ITS, US stock exchanges operated almost independently. The New York Stock Exchange dominated, but the American Stock Exchange, Pacific Stock Exchange, Midwest Stock Exchange, and Philadelphia Stock Exchange all quoted and traded the same stocks. A buyer on one exchange might pay $50 while the same stock quoted at $49.50 elsewhere—and nobody was required to bridge that gap.

ITS emerged from regulatory pressure to fix this fragmentation. The Securities and Exchange Commission wanted one national market. Beginning in 1978, participating exchanges installed a central messaging system that displayed the best bids and offers across venues in real time. The system required members to post their quotes electronically and, crucially, to deliver orders to whichever exchange had the better price—the core principle of avoiding “trade-throughs.”

The technology was deliberate: slow by modern standards but revolutionary then. A broker seeing a better quote on another exchange could route the order automatically through ITS, rather than executing locally at an inferior price. For the first time, equity traders faced genuine price transparency across the country.

The Trade-Through Rule and Its Limits

The heart of ITS was its trade-through rule: a broker should not execute an order at a given price if a better price was publicly quoted on another exchange. This sounds clean in theory. In practice, it had teeth only if exchanges complied—and they had little incentive to lose order flow to competitors.

ITS never mandated best-price execution. Instead, it relied on industry self-regulation and SEC surveillance. If a broker at the NYSE saw a better offer posted on the Philadelphia Exchange, ITS would display it and allow the broker to route the order away. But if the broker chose to execute locally anyway, the penalties were soft: the exchange could fine the firm or suspend access, but enforcement was inconsistent. Major firms often negotiated exemptions or found loopholes.

Furthermore, NASDAQ—the decentralized electronic market where most technology stocks traded—joined ITS late and awkwardly. Market makers on NASDAQ were not mandatory participants in the linkage, creating two distinct tiers of price transparency. A buyer might see one price on the NYSE and a different one in the NASDAQ market maker system, with no automated bridge between them.

Why ITS Eventually Became Insufficient

By the early 2000s, market structure had changed radically. Electronic Communications Networks (ECNs) had emerged, offering faster, cheaper execution than traditional exchanges. Retail trading volumes exploded. The definition of what qualified as a “quotation” under ITS became contested—did a market maker’s internal indication count? What about orders posted on an ECN but not advertised to all?

Fragmentation had actually worsened. Traders could execute on dozens of venues, and ITS’s central messaging system could not keep pace with the speed of modern trading. Its automation helped, but it still lagged behind the millisecond world that electronic venues now inhabited.

Enforcement remained spotty. Large brokers could sustain violations and negotiate settlements; smaller firms faced harsher treatment. The rule was also susceptible to gaming: an exchange could widen its spread to avoid triggering trade-through obligations, or a market maker could post tiny sizes at good prices to avoid serious risk.

Regulation NMS: The Successor Framework

In 2005, the SEC adopted Regulation NMS (National Market System Regulation), which formally replaced ITS with a more comprehensive set of rules. The shift reflected a decade of debate over what a modern national market should require.

Reg NMS retained the trade-through protection but applied it more uniformly across all venues—exchanges, ECNs, and market makers. It mandated that if a broker received an order, and a better-priced quotation was displayed on another venue, the broker must either (1) execute at the better price or (2) route the order to that venue. No longer could exchanges act as filters; the order protection rule applied uniformly.

Reg NMS also introduced the access rule, which required all trading centers to provide fair and efficient market access, preventing any exchange or ECN from bottlenecking order flow. This made fragmentation itself acceptable—as long as orders were routed to the best price regardless of which venue had it.

The new framework was faster, more rule-based, and harder to circumvent than ITS had been. But it came with new complications: latency, complexity in order routing, and the occasional execution mishap when market makers or systems failed to honor their obligations.

What ITS Teaches About Market Linkage

The Intermarket Trading System reveals a pattern in market evolution: regulation trails technology and complexity. ITS worked reasonably well when trading was slower, exchanges fewer, and the ecosystem more homogeneous. As markets accelerated and fragmented, the system’s gaps became critical.

ITS also shows the tension between unified pricing and individual exchange autonomy. Exchanges feared that a too-strict national rule would weaken their competitive position. Yet without enforcement with teeth, the national market remained notional, and retail and institutional traders paid the cost.

Regulation NMS shifted the balance decisively toward the national market concept, even as it permitted fragmentation. Whether that trade-off has succeeded remains debated—modern markets are faster and cheaper but also more prone to sudden glitches and flash crashes.

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