# Order book

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Canonical: https://novigapi.com/learn/order-book

An order book is the list of resting buy orders on an exchange market, organized by price, showing how many contracts traders will trade at each level.

An order book is the live record of every resting order on a market. Each line shows a price and the quantity waiting there. The best bid is the highest price someone will pay. The book's shape, how much sits at each level, tells you how much you can trade before the price moves against you. Reading the book is the difference between knowing a market's quoted price and knowing what you would actually pay.

## How it works

Orders sit in price-time priority: better prices first, and at the same price, earlier orders first. A taker's order fills against the best resting price, then the next, until it is complete. Each level it eats through is called sweeping the book, and the average price paid rises with each level. Most exchanges show two sides, bids and asks. Novig's book is built differently.

## On Novig

Novig's book holds bids only. On a two-outcome market, traders post bids on Yes and bids on No. To buy Yes now, you match a resting No bid: a No bid at 42% is a Yes offer at 58%. So the Yes ask at each level is 1 minus a No bid, and the quantity at that ask is the quantity behind the No bid. Novig's legacy API docs describe the public book as partly obfuscated. Our `/orderbook/{id}` endpoint returns both sides with the implied asks already computed.

> Units: this page counts a contract as $1.00 of payout, as Novig's [fees page](https://docs.novig.com/fees) does. In Novig's v3 API one contract pays 1 cent, so each $1.00 unit here is 100 API contracts; divide API quantities by 100 before using these numbers.

## Worked example

A bids-only book and the Yes asks it implies

| Level | Yes bid | Yes qty | No bid | No qty | Implied Yes ask |
| --- | --- | --- | --- | --- | --- |
| 1 | 55% | 500 | 42% | 800 | 58% |
| 2 | 54% | 1,200 | 41% | 1,500 | 59% |
| 3 | 52% | 3,000 | 40% | 2,500 | 60% |


To buy 2,000 Yes contracts now, you take all 800 at 58% and 1,200 of the 1,500 at 59%: 800 x $0.58 + 1,200 x $0.59 = $464 + $708 = $1,172, an average of 58.6%. Buying all 4,800 contracts on the No side's book would cost $2,849, an average of about 59.35%. If the event is live, each fill also pays the taker fee at its own price.

```
average price = sum(qty_i x price_i) / sum(qty_i) = 1,172 / 2,000 = 0.586
```

The [Novig odds converter](https://novigapi.com/novig-odds-converter) turns any of these percentages into American and decimal odds.

## Common mistakes

- Assuming you can trade any size at the top price. Only the quantity at that level is available.
- Reading Novig's bids as asks. A Yes bid at 55% is someone wanting to buy, not sell; the Yes ask comes from the No side.
- Trusting a book just before the start. Novig's docs say resting orders are voided when an event goes live.

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