SIE Ready🔁 Market Structure · Lesson 4 of 4 · 5 min
ATSs, ECNs and Dark Pools
🌑 A fund manager needs to sell 2 million shares. Post that order on an exchange and the price drops before she sells a tenth of it. That is why dark pools exist.
💡 Key idea
ATSs are broker-dealer-run electronic venues, not exchanges. ECNs match orders automatically; dark pools hide orders before execution so institutions can move large blocks quietly.
🧠 Why it matters
An alternative trading system (ATS) is an SEC-regulated electronic venue that matches buyers' and sellers' orders like an exchange but is not registered as one. Under the SEC's Regulation ATS, it operates as a registered broker-dealer instead. An electronic communications network (ECN) is an electronic system that automatically matches orders; ECNs also register as broker-dealers under Regulation ATS, and they made direct institution-to-institution (fourth market) trading easy. A dark pool is an ATS that does not publicly show the size and price of its orders before execution. Institutions use dark pools for large block trades to limit price impact and information leakage. Trades in dark pools are still reported after they happen. The trade-off: less pre-trade transparency for the wider market. Block trades are simply very large trades, usually by institutions.
📈 In the real world
🏢 An insurance company's trader routes a 300,000-share order to a dark pool. Over the morning it fills in pieces against other institutions at prices inside the public quote, and nobody watching the exchange saw the order coming. After each fill, the trade is reported and appears in the public data.
📌 Takeaways
- An ATS matches orders electronically but registers as a broker-dealer, not an exchange
- ECNs match orders automatically and also operate under Regulation ATS
- Dark pools hide orders before execution to help institutions trade blocks with less price impact
📖 Terms in this lesson
Alternative trading system (ATS): A trading venue that matches buyers and sellers like an exchange but operates under SEC Regulation ATS instead of registering as an exchange.
Dark pool: An ATS that does not display orders publicly before execution, often used by institutions to trade large blocks quietly.
Electronic communications network (ECN): An electronic system that automatically matches buy and sell orders at specified prices.
✅ Test yourself
How is an alternative trading system regulated by the SEC?
- As a broker-dealer under Regulation ATS
- As a national securities exchange
- As an investment adviser
- As a self-regulatory organization
Answer: A · As a broker-dealer under Regulation ATS
Regulation ATS lets these venues register as broker-dealers and file Form ATS instead of registering as exchanges. Because they are not exchanges, they are not SROs, and matching trades is not investment advice.
What feature defines a dark pool?
- Trades are never reported to anyone
- Only retail investors may participate
- It is operated by the Federal Reserve
- Orders are hidden before they execute
Answer: D · Orders are hidden before they execute
Dark pools do not show orders to the public before execution, which helps institutions trade large blocks. Executed trades are still reported, users are mainly institutions, and dark pools are run by broker-dealers, not the Fed.
Why do institutions most often use dark pools?
- To trade listed options without going through the OCC
- To avoid having to report their trades at all
- To buy new issues before an IPO
- To trade large blocks with less price impact
Answer: D · To trade large blocks with less price impact
Hiding a large order before execution limits price moves and information leakage. Dark pool trades are still reported, IPO allocations happen in the primary market, and listed options clear through the OCC.
SIE-style questions, XP and streaks in the app. No sign-up needed.