Trading and Exchanges Chapter 15: How Big Trades Get Done Without Crashing Prices
Say you manage a pension fund and you need to sell 500,000 shares of some stock. You cannot just drop a market order on the exchange. The order book does not have that much liquidity sitting around. If you try to force it through, you will eat through every level of the book and crash the price on yourself. Chapter 15 is about how these giant trades actually get done.
What Counts as a Block Trade?
A block trade is any order too large to fill easily through normal trading. Harris says most block traders think of it as anything exceeding a quarter of a day’s average volume. The NYSE officially defines a block as 10,000 shares or more, but that number is arbitrary. For a liquid mega-cap, 10,000 shares is nothing. For a thinly traded small-cap, 2,000 shares might be impossible to move.
The real definition is practical: if you cannot fill it without significant price impact, it is a block.
The Four Problems of Moving Big Size
Harris identifies four problems that make block trading hard. This is the core of the chapter.
The latent demand problem. Most potential counterparties are not in the order book. They might trade if someone asks, but they have not placed orders. This liquidity exists in theory but not in practice. Someone has to go find it.
The order exposure problem. To find buyers, you have to tell people you are selling. But the moment word gets out, front runners jump ahead of you. Traders on your side accelerate their selling. Traders on the opposite side hold back their buying. Harris calls this “spoiling your market.” The more you shop the block, the worse your price gets.
The price discrimination problem. Liquidity suppliers are scared you will slice your order into pieces. Say you tell someone you are selling 200,000 shares. They give you a fair price. Then you sell another 200,000 to someone else, pushing price down further. The first buyer just lost money. So everyone wants to know your real total size before they commit.
The asymmetric information problem. Everyone suspects big traders know something. Large traders can afford better research, and informed traders want to trade big to maximize their edge. So when you show up with a massive order, the other side assumes you are probably informed. They either refuse to trade or demand a huge price concession.
The Upstairs Market
This is where block traders come in. They operate in what Harris calls the “upstairs market,” named because trades were arranged in offices above the NYSE trading floor.
Block dealers (block positioners) take the other side of your trade with their own capital. They buy your 500,000 shares, then slowly work out of the position over time. Real risk, so they need to be confident you are not informed.
Block brokers (block assemblers) do not take positions. They call around and assemble enough buyers to absorb your sell order. Harris compares this to the card game Concentration. They need to remember who is interested in what, who traded recently, who might buy at the right price. Good block brokers spend their entire day on the phone.
Both types solve the four problems by adding trust and investigation. They selectively expose orders to trustworthy counterparties. They audit motives. They verify order sizes. They put their reputation and sometimes their capital on the line.
Trust Is Everything
Block trading runs entirely on reputation. If you lie about your order size, you end up “in the doghouse.” Harris gives a great example: Blair tells broker Sawyer he wants to buy 200,000 shares of IBM. Sawyer sells at a small premium. Then Blair immediately buys another 200,000 from someone else at a higher price. Sawyer lost $100,000. He will never willingly trade with Blair again.
Front runners get punished too. If a broker shows you an order and you trade ahead of it, you lose access to future blocks. And those blocks are profitable since initiators give real price concessions to attract liquidity.
Block traders also conduct what Harris calls a “trading motive audit.” A pension fund selling because of a regulatory requirement? Clearly uninformed, happy to trade with you. A hedge fund selling right before earnings? Very suspicious.
A Telling Asymmetry
About 80% of large block trades in U.S. stocks are seller-initiated. Why? Three reasons. Sellers can sell to anyone, but buyers can only buy from current holders. Sellers can credibly prove they have no more to sell (you can check their holdings). And sellers have better “uninformed” stories: need cash, regulatory limits, portfolio rebalancing. A buyer targeting one specific stock in huge quantity just looks informed.
Why This Matters Today
Dark pools evolved from exactly these problems. They let institutions match large orders without exposing them to the broader market. The challenges Harris describes from the phone-and-handshake era are the same challenges electronic venues solve today.
The core lesson: the act of trading itself reveals information. The bigger you trade, the more you reveal. Block traders, whether human or algorithmic, exist to manage the tension between finding liquidity and protecting intentions.
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