How Wall Street Works
Market Makers and Liquidity: Who Is on the Other Side of Your Trade
Someone takes the other side of every trade you make, and they are paid to do it. What a market maker earns, why the spread widens when you most want to sell, and how to measure the toll.
So you bought 200 shares at 10:14 on a Tuesday and it felt like the market sold them to you. It did not. A firm sold them to you, a firm whose entire business is standing in the middle with two prices showing, ready to trade with anybody who arrives in either direction. It did that because it is paid to. The payment is the bid-ask spread, and you are the one making it.
What the firm has actually promised
A market maker shows two prices all session. A bid, the price at which it will buy your stock from you, and an ask, the price at which it will sell stock to you. The ask is always higher.
It has no view on the company. The plan is to buy at the bid, sell at the ask, and finish the day close to flat, having taken the difference several hundred thousand times.
Quote 49.99 bid, 50.01 ask. Buy 200 shares from a seller and sell 200 shares to a buyer within a few seconds and the firm earns 200 x $0.02 = $4.00. The risk, and it is a real one, is that only one side of the market shows up and the firm ends the hour holding 200,000 shares into a falling price.
Inventory risk
A market maker that buys your shares owns them. The price can move before the offsetting sale arrives, and the more the stock jumps around, the more compensation the firm demands for holding it, which is why spreads widen the moment volatility rises. No decision was made about you.
Adverse selection
The second cost sets spreads in the stocks where it hurts. Some of the people trading against a market maker know something. If a fund is quietly unloading 2 million shares ahead of a downgrade, every fill the market maker takes on the bid is a loss on arrival. The firm cannot tell which orders are informed until afterwards. So it charges everyone in advance.
The wholesalers your order never gets past
Most U.S. retail stock orders never reach a public exchange. Your broker routes them to a wholesale market maker, one of a handful of firms that specialise in filling retail flow internally, and the wholesaler pays the broker for doing it.
That is where payment for order flow gets its name. It is the main reason stock commissions went to zero.
The logic is adverse selection again. A retail investor buying 150 shares is far less likely to be trading ahead of news than a 400,000 share institutional order, so the flow is cheaper to service, so the wholesaler can quote inside the public spread and still make money.
Price improvement, priced
The national best bid and offer is 49.99 by 50.01. Your buy order fills at 50.005. On 200 shares you saved 200 x $0.005 = $1.00 against the public ask. That saving is genuine. It is also the smallest of the three outcomes available to you.
| Where a 200 share buy is filled | Price paid | Cost against the 50.00 midpoint |
|---|---|---|
| Public ask, no improvement | 50.01 | 2.00 |
| Wholesaler with half-cent improvement | 50.005 | 1.00 |
| Patient limit order resting at the bid | 49.99 | you earn 2.00 |
The bottom row is the point of this section. A limit order that rests in the book and gets hit is paid the spread, a four dollar swing on 200 shares against the top row, and what you give up is any certainty that it fills at all. Order types explained covers how to place one, and when the missed fill costs more than the spread you saved.
Liquidity in three numbers
People use liquidity as a vague compliment. Three figures make it concrete.
Spread, expressed as a percentage of the price. Two cents on a 50 dollar stock is $0.02 / $50 = 0.04%. Two cents on a 4 dollar stock is $0.02 / $4 = 0.5%, twelve and a half times as expensive for the same dollar exposure. The cent figure tells you nothing on its own.
Depth, meaning the shares sitting at the best bid and ask. A quote of 49.99 by 50.01 with 200 shares a side is a thin market wearing a tight spread, and your 1,000 share order walks straight through it into the next price level.
Average daily volume, which sets the ceiling on your position. Keep a single order under 1 percent of typical daily volume and you will rarely be the reason the price moved, so on a stock trading 300,000 shares a day that is 3,000 shares, and anything larger needs breaking up or placing with limits.
What it costs you over a year
Take a 200 share position in a 50 dollar stock, bought and sold once.
One cent spread, the round trip against the midpoint is 200 x $0.01 = $2. Five cents, 200 x $0.05 = $10. Twenty five cents, the kind of quote you see in a small company or after hours, 200 x $0.25 = $50 on a $10,000 position. That last one is half a percent gone before the price does anything.
Now add frequency. That is where the damage lives. Four round trips a month in that five cent stock is 4 x 12 x $10 = $480 a year on a 10,000 dollar position, a 4.8 percent annual drag charged silently, with no line item and no confirmation email, and every strategy you run has to clear that hurdle before it earns a dollar.
What the folklore gets wrong
The story retail traders tell is that a firm somewhere is hunting their stop. It is comforting. It makes the loss somebody’s decision.
What actually happens is duller and costs more. When volatility spikes, market makers are obliged to quote and they are not obliged to quote size at a good price, so the rational move is to widen and shrink. That is why the worst executions in your account cluster in the worst minutes. On a gap down open, spreads run several times their normal width, and a market order sent into that gets a price nobody would have accepted an hour earlier. The limit up limit down rules pause a stock that moves outside a band in a five minute window, which caps the damage without removing it.
Nobody looked at your account. The quote thinned for everyone at once and your order arrived during the thin part. The same thinning runs all day outside the regular 9:30 to 4:00 Eastern session, covered in stock market hours, and takes more sophisticated forms in dark pools, HFT and Reg NMS and market microstructure.
That comparison is the whole discipline. Learn order types so you can choose which side of the spread you are on, read what is Wall Street for how these firms fit the wider system, and check average volume in the stock screener before you commit to a thin name. The How Wall Street Works quiz will tell you whether the mechanics stuck.
Frequently asked questions
What does a market maker do?
A market maker quotes a price at which it will buy and a price at which it will sell, continuously, in a given stock. That commitment lets you trade immediately instead of waiting for a natural buyer or seller to appear. The firm earns the difference between its two prices, and carries the risk of holding inventory that moves against it.
Why does the bid-ask spread exist?
The spread compensates the market maker for two costs. One is inventory risk: holding shares that can fall in value before they are sold. The other is adverse selection, the risk that whoever is trading with them knows something they do not. Stocks with heavy volume and stable prices carry narrow spreads, and thin or volatile stocks carry wide ones.
What is payment for order flow?
Wholesale market makers pay brokers to send them retail orders. The wholesaler fills those orders itself, usually at a price slightly better than the public quote, and keeps part of the spread. It is why most U.S. brokers charge no commission on stock trades. Regulators require brokers to disclose these arrangements and their execution quality.
Is payment for order flow bad for me?
It is a trade. You give up commission charges and typically receive a small amount of price improvement on the fill. You also lose the chance to interact with the public order book, where a patient limit order could sometimes do better. For small orders in liquid stocks the difference is measured in fractions of a cent per share.
What is liquidity in simple terms?
Liquidity is how much you can trade, how quickly, without moving the price against yourself. A stock where 50,000 shares trade at the touch of a button with a one cent spread is liquid. A stock where 500 shares move the price 2 percent is not, regardless of how good the company is.
Do market makers push prices against retail traders?
The common version of that story, where a firm hunts an individual stop loss, does not match how these systems work at scale. What is true is duller and more expensive: spreads widen when volatility rises, quoted size shrinks, and orders that demand immediacy pay more. You lose money to the mechanics rather than to a plot.