Block Traders: How Large Orders Move Through Markets (Chapter 15)
Book: Trading and Exchanges: Market Microstructure for Practitioners Author: Larry Harris Publisher: Oxford University Press, 2003 ISBN: 0-19-514470-8
Imagine you need to sell 500,000 shares of a stock that normally trades 200,000 shares a day. You can not just hit the sell button and walk away. You would destroy the price. Chapter 15 of Harris’s book explains how large orders actually get executed, and it is a world of phone calls, trust, and careful detective work.
What Makes a Block a Block
A block trade is any order too large to fill easily through normal market channels. Harris defines it practically rather than by some fixed number. Most block traders think of a block as anything larger than about a quarter of a day’s average trading volume.
The New York Stock Exchange technically classifies anything over 10,000 shares as a block trade, but in actively traded stocks, 10,000 shares is nothing. For thinly traded stocks, even a few thousand shares might be a genuine block. The official definition is kind of useless. What matters is whether the order overwhelms available liquidity.
Block trades are a small fraction of total trades by count, but they account for a huge chunk of total volume because of their size.
Four Problems That Make Blocks Hard
Block initiators face four distinct problems, and each one is serious.
The latent demand problem. Most potential liquidity suppliers are not sitting there with orders in the market. They would trade if asked, but they have not bothered to submit orders. Maybe it is too costly to manage a bunch of standing orders. Maybe they have not even thought about trading that particular stock today. A trader might be willing to buy hundreds of different stocks at prices 5 percent below market, but he can not have orders for all of them. What if the whole market drops and every order fills at once?
Block traders solve this by working the phones. They keep track of who trades what, who has expressed interest before, and who might be persuaded to participate. Good block traders play a game similar to the card game Concentration, constantly remembering who might be interested in trading hundreds of different securities.
The order exposure problem. When you are trying to fill a large order, you have to show it to people. But every person who sees your order is a potential front-runner. If word leaks that someone is trying to sell 500,000 shares, traders will rush to sell ahead of you, pushing the price down before you can execute. Your market gets spoiled.
Block traders “shop the block” carefully, showing it first to the most trustworthy counterparties. Traders who front-run or leak information get put in the doghouse. They lose access to future block trades, which can be very profitable to participate in.
The price discrimination problem. Liquidity suppliers are afraid that a large trader will break their order into pieces and discriminate on price. Say a block seller gets you to buy 200,000 shares, then immediately sells another 200,000 shares to someone else, pushing the price down further. You just lost money because you did not know about the full order.
To solve this, block traders need to credibly reveal the total size of their orders. This requires trust and reputation. Block brokers who are caught lying about order sizes lose their business. Lying has serious consequences because the block trading world is small and reputation matters enormously.
The asymmetric information problem. Liquidity suppliers suspect that anyone trading a huge block might know something. Big traders can afford better research. Informed traders want to trade large to maximize profits from their information. So when someone shows up with a massive order, the natural assumption is that they might be informed.
Block initiators solve this by revealing their identities and trading motives. If you are an index fund rebalancing, that is clearly uninformed. If you are selling because ERISA rules force your pension plan to reduce a position, that is verifiable. But if you are an actively managed hedge fund with no obvious reason to trade, good luck getting someone to take the other side.
The Upstairs Market
Most large block trades happen “upstairs” rather than on the exchange floor. The name comes from the old days when trading desks were literally in offices above the New York Stock Exchange trading floor.
The upstairs market works through two types of block traders.
Block dealers (also called block positioners or facilitators) take the client’s position themselves. They buy the block from a seller, hold it, then try to trade out of it over time. This is risky. If prices move against them before they can sell, they lose. Block dealers need to be very confident that their clients are uninformed and honest about the total order size.
Block brokers (also called block assemblers) find other traders willing to take the opposite side. They assemble the block from multiple participants. This is less risky for the broker personally, but their reputation is on the line. If the traders they brought into the deal end up losing because the block initiator was informed, those traders will never work with that broker again.
In practice, most block traders do both. Broker-dealers can serve clients better because they can do whatever is needed at the moment. And when a broker is willing to put their own money at risk alongside other participants, it sends a stronger signal that the trade is safe. Harris compares this to a magician who puts their own finger in the guillotine alongside the volunteer’s hand.
The Trading Motive Audit
Before filling a block, traders research why the client wants to trade. The ideal client has a clear, verifiable, uninformed reason. A pension fund selling employer stock to comply with ERISA limits. An index fund rebalancing after a reconstitution. A mutual fund raising cash to meet redemptions.
Harder cases involve clients who have some discretion in what they trade or when they trade. If a client could have sold any stock but chose this particular one, the block trader might wonder if the client knows something specific.
Some people are skeptical that block brokers actually audit trading motives effectively. In volatile markets, it can be hard to tell whether bad performance after a block trade was related to the trade itself or just random price movement. If brokers can not build a track record of protecting their clients, the whole system of trust breaks down.
Why 80% of Blocks Are Seller-Initiated
Here is a fascinating statistic. About 80 percent of large block trades in U.S. stock markets are initiated by sellers, not buyers. Three of the four block trading problems explain why.
Sellers can sell their blocks to any interested buyer. Buyers face a harder search because they generally need to find someone who actually owns the specific stock, especially if short selling is difficult.
Sellers can more credibly reveal the full size of their orders. If you own 500,000 shares and can not sell short, your maximum order is 500,000 shares. Buyers have no such natural limit.
Sellers generally have more convincing stories about why they are uninformed. They need cash, they are reducing a concentrated position, they are rebalancing. Buyers who want to load up on one specific stock look suspicious. Why this stock? Why now?
Block Markets and Regular Markets
Block trading creates regulatory headaches because trades arranged privately might conflict with orders standing in the regular market. The U.S. equity exchanges address this with a size precedence rule. For crosses over 25,000 shares, matched block orders can jump ahead of orders with greater time precedence. But any standing orders at better prices must still be filled at the block price.
This is a compromise. Exchanges want to protect block traders who worked hard to assemble their trades. But they also want to protect limit order traders who provide liquidity to the regular market. Breaking up arranged blocks would discourage block trading. Ignoring standing orders would discourage limit order submission.
The Trust Economy
What stands out most about block trading is how much of it runs on trust and reputation. Anonymous electronic markets are great for small orders, but they can not solve the problems that block traders face. You can not verify someone’s trading motives through a screen. You can not build a reputation for honesty in an anonymous system.
Block trading markets work because the same people trade with each other repeatedly. Lying has real consequences. Front-running gets you blacklisted. Dishonesty costs you access to profitable future trades.
In a world that is increasingly electronic and anonymous, the old-school phone-based block trading market is a reminder that sometimes relationships and reputation are the most efficient technology available.
This is part of a series on Larry Harris’s “Trading and Exchanges: Market Microstructure for Practitioners.” The book is dense and technical, but it explains the real mechanics behind how markets work. If you trade anything, it is worth your time.