Module 1 · Lesson 3
How contracts work
Behind every price you’ve read is a live, two-sided list — the order book. This lesson opens it up: the bid, the ask, the spread between them, and why the price you see isn’t always the price you get.
Key takeaways
- Behind every price is an order book — a live list of what buyers will pay and what sellers will accept. The price you’ve been reading is just where those two sides most recently met
- There are always two prices, not one: the bid (the highest anyone will pay) and the ask (the lowest anyone will sell for). The gap between them is the spread
- Liquidity is how much you can trade without moving the price. A liquid market has a tight spread and deep orders; a thin one has a wide spread and moves the moment you touch it
- The price you see and the price you get are not always the same. Understanding why is the difference between reading a market and trading in one
The number was hiding something
For two lessons, we’ve talked about “the price” as if it were a single number. A contract is “at 65 cents.” The market “believes 65%.” That was true enough to build the ideas on — but it was a simplification, and now it’s time to look at what the number was hiding.
There is no single price. At any moment, a prediction market contract has two prices: the most anyone is currently willing to pay for it, and the least anyone is currently willing to sell it for. The “65 cents” you saw was shorthand — usually the price of the last trade that happened, or the midpoint between those two standing offers. The real machinery underneath is a list. This lesson is about that list, because once you can read it, you can see things about a market that the headline number hides completely.
The order book
Every serious market — for stocks, currencies, prediction contracts — runs on an order book: a live, two-sided list of intentions.
On one side are the buyers, each saying “I’ll buy this contract at this price, for this many shares.” On the other are the sellers, each saying “I’ll sell at this price, for this many.” The book stacks these up in order — the best offers on each side at the top.
Imagine the order book for a “USA wins” contract:
| Buyers want to pay (bid) | Sellers want to receive (ask) | |
|---|---|---|
| best | 64¢ × 200 shares | 66¢ × 150 shares |
| next | 63¢ × 500 shares | 67¢ × 400 shares |
| next | 61¢ × 1,000 shares | 69¢ × 800 shares |
| next | 58¢ × 1,500 shares | 72¢ × 1,600 shares |
Read the top row. The highest anyone will pay right now is 64¢ — that’s the bid. The lowest anyone will sell for is 66¢ — that’s the ask. Nobody is trading at 65¢, even though that might be the number you’d see quoted, because 65¢ is just the midpoint between the two sides. It’s a price at which, right now, no actual order exists.
This is the first thing the single number hid: the “price” is often a point where nobody is actually offering to trade.
The spread
The gap between the bid and the ask — here, 64¢ to 66¢ — is the spread. It is two cents wide.
The spread is not an accident or a fee someone charges. It emerges naturally from the fact that buyers want to pay less and sellers want to receive more. It’s the no-man’s-land between the two sides of the book, and it matters for a very concrete reason: it’s the cost of trading immediately.
Suppose you want to buy a “USA wins” contract right now, this instant. You can’t buy at the bid — that’s what buyers are offering, and you’re a buyer too. To buy immediately, you have to accept the best ask: 66¢. Now suppose you change your mind a second later and want to sell immediately. You can’t sell at the ask; you have to accept the best bid: 64¢. You just lost two cents — the spread — for the privilege of going in and out instantly.
That two cents is the toll for immediacy. And it connects directly to the last lesson: the spread is a measure of liquidity.
Buying immediately at whatever the book offers is called a market order — it guarantees you get the size you asked for, at whatever price the book charges. The alternative is a limit order: you name your price and wait. A limit order never pays the spread, but it may never fill at all. Size or price — you can have one guaranteed, not both. We’ll return to order types when we look at market structure.
Liquidity: how much the market can absorb
Liquidity is the single most important property of a market that beginners ignore. It’s the answer to one question: how much can you trade before you move the price?
Look back at the order book. If you want to buy just 150 shares, you can take the entire best ask (66¢ × 150) and you’re done — you paid 66¢. But if you want to buy 500 shares, 150 aren’t enough. You take all 150 at 66¢, then the next 350 come from the next level up — 67¢. Your average price is worse than the number you first saw. Buy 2,000 shares and you’ll eat through several levels, pushing the price up as you go. This is called slippage: the price slips away from you as your own order consumes the available offers.
Buy "USA wins" — how much does your own order cost you?
The book shows what buyers will pay and what sellers will accept, right now. Drag the size and watch which offers your order consumes.
This is a market order — it fills immediately, taking whatever the book offers, which is why the average price worsens as the order grows. A limit order would name your price and stop there: you would get 150 contracts at 66¢ and nothing more, rather than 500 at a worse average. Size or price — you can guarantee one, not both.
This book holds 2,950 contracts across four price levels — that is what "depth" means, and it is the whole story of whether your order fills cleanly. A simplified snapshot: real books have many more levels and refill continuously as new orders arrive. Liquidity varies enormously between markets — a heavily traded contract may absorb thousands without moving, while a quiet one runs out in a few hundred.
A liquid market — thousands of participants, orders stacked deep at every level, a spread of a cent or less — barely flinches when you trade. You can move real size and the price hardly notices.
A thin market — a handful of traders, gaps between price levels, a spread of five or ten cents — moves the instant you touch it. Your own modest order becomes the news.
Now the failure mode from Lesson 2 should click into place. We said thin markets are unreliable and easily manipulated. The order book shows you why: in a thin market, there’s almost nothing standing in the book, so a single order — yours, or a manipulator’s — walks the price wherever it wants with very little money. Liquidity is the thing that makes a price trustworthy, and the spread is how you spot it at a glance. Wide spread, few orders: distrust the number. Tight spread, deep book: the number means something.
The price you see vs. the price you get
Put it together and you arrive at the practical truth this lesson exists to deliver.
The price displayed on a prediction market — the “65%” in the headline, on the news chyron, in the screenshot someone posts — is a single, clean number. But underneath it is a two-sided book with a spread and a finite depth. Which means:
- The displayed price is usually the last trade or the midpoint. It’s a summary.
- The price you’d actually pay to buy right now is the ask, which is higher.
- The price you’d actually receive to sell right now is the bid, which is lower.
- And the price you’d get for a large order is worse still, because you’d move through the book.
For reading a market as a forecast — which is most of what these first lessons have been about — the displayed midpoint is fine. A market saying 65% is telling you something real about the world, and the spread is a rounding error on that signal.
For trading in a market, the spread and the depth are not a detail — they’re most of the game. The difference between a one-cent spread and a ten-cent spread is the difference between a market you can move in and out of freely and one that charges you a toll every time you change your mind.
Buy it, then change your mind one second later.
The market doesn't move. No news, no time passes. You buy your contracts and immediately sell them back. Watch what it costs you.
Both legs here are market orders — buy now, sell now, at whatever the book offers. That is what makes the round trip cost you: immediacy is the thing you are paying for. A patient trader using limit orders on both sides could avoid the spread entirely, at the risk of never filling at all.
A simplified snapshot. Real order books have many more price levels and refill continuously as new orders arrive — so a large order often fills better than a still picture suggests. Liquidity also varies enormously between markets: a heavily traded contract may absorb thousands without moving, while a quiet one runs out in a few hundred.
You don’t need to trade to benefit from knowing this. Even as a pure reader, the spread and depth tell you how much to trust the price — which is exactly the critical-reading skill Lesson 2 was building. Now you have the tool that makes it concrete.
The bottom line
The single price you’ve been reading is the visible tip of a two-sided order book. The bid is the most anyone will pay; the ask is the least anyone will sell for; the spread between them is the cost of trading immediately and a live readout of the market’s liquidity. A tight spread over a deep book means a price you can trust and trade against; a wide spread over a thin book means a number that moves the moment anyone leans on it.
That’s the machinery. You now know what a prediction market is (Lesson 1), why its prices can be trusted (Lesson 2), and how the contracts actually change hands (Lesson 3).
Next, we step back from the mechanics and look at the landscape: who runs these markets, what you can actually trade on them today, and how the major platforms differ.
Related lessons
Previous in Module 1:
- Lesson 2: Why markets can predict — The wisdom-of-crowds mechanism that lets prices aggregate dispersed information, and where it breaks down.
Next in Module 1:
- Lesson 4: The current landscape — Kalshi, Polymarket, sportsbooks, and forecasting platforms. Where prediction markets sit in the broader ecosystem.