Module 1 · Lesson 3
The order book, order entry & the tape (Time & Sales)
Behind every price you’ve read is a live, two-sided list of buyers and sellers — the order book. This lesson opens it up, shows how you place a trade through order entry, and how every trade prints to the tape. Along the way the single “price” turns out to be several — and you’ll see why the price you see isn’t always the price you get.
- Read an order book — bid and ask, the spread between them, and the depth stacked on each side
- Use order entry to place a market or limit order, and predict whether it fills now or rests in the book
- Read the tape (Time & Sales) to see what actually traded, at what price and size
- Explain slippage — why the price you get can differ from the price you saw, and when it bites hardest
There isn’t one price
Here’s a market: USA to win the World Cup basketball final, trading at 65¢. One clean number. It looks like the price — the way a scoreboard shows you one figure and moves on.
But depending on the market and prediction-market platform, that displayed price might be the last traded price, the bid, the ask, or the midpoint — the average of the bid and the ask, which some platforms display as the market’s probability.
The order book
What looks like one price makes more sense once you open the order book. It shows not only where the price you saw comes from but also the broader market behind it — the bid and the ask, the spread, the depth, and the liquidity. Select a concept to see where it sits. On the Liquidity tab, you can place a test order and watch it walk the book.
Select Buy or Sell, enter an amount in contracts or dollars, and press Fill it. The highlighted orders are the ones your trade takes, and the result shows your average price and any slippage.
Order entry
Now that you can read the book, here’s the order form you place trades with. Select a tab to see each part highlighted and set — the direction, the side (Yes or No), the type, the amount. The Place order tab makes the form live: pick a liquidity level, place a market order, and watch the slippage it costs you (and how a limit order avoids it).
The tape (Time & Sales)
Every order that actually fills leaves a record — the tape, or Time & Sales. It’s the running list of executed trades: what traded, when, on which side, at what price, and for how much. Submit a trade and watch it print. Notice there’s no buy/sell column — the tape records only the taker’s side (Yes or No), because on binary exchanges like prediction markets, selling one side is the same trade as buying the other.
Select Buy or Sell and a side, enter an amount, then press Submit trade. Your trade prints as a new row at the top of the tape.
Putting it all together
Here’s the whole thing working as one live market. Place an order in the order entry and watch it hit the order book and print to the tape at the same moment. Switch between events to feel how liquidity changes everything — a market order on a low-liquidity market of the Friendly has more slippage than the same order on a high-liquidity market of the World Cup.
One shared book drives all three panels — market orders consume and print, limit orders rest and wait.
The price you see is the top of a stack
Two prices — the bid and the ask — with the spread between them (Level 1). Behind them, the depth of the book (Level 2) shows you the market’s liquidity. You act through order types — a market order (Market Buy or Market Sell) takes liquidity and can sometimes fill at a different, less favorable price than expected, which is slippage; a limit order (Limit Buy or Limit Sell) rests and provides liquidity, making you a maker.
Every fill prints to the tape — the running record of what actually traded. Read the book to see what’s available; read the tape to see what happened. Put together, they’re the full picture of a market in motion.
You’ve now seen the order book that sits behind a single price — the bid, the ask, and the depth that determines what your trade really costs. Next: where all of this runs — the platforms, how they differ on regulation, access and funding, and the same machinery under them all →
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.