> ## Documentation Index
> Fetch the complete documentation index at: https://docs.rails.xyz/llms.txt
> Use this file to discover all available pages before exploring further.

# Market Data

> Contracts, quotes, order books, and trades

Market data is delivered over WebSocket streams — see each section below for the specific
stream — with a small REST surface for discovery via the
[Get Expirations](/latest/options/rest-api/get-expirations) and
[Get Contracts](/latest/options/rest-api/get-contracts) endpoints.

## Contracts

A **contract** is a single tradable option, identified by its
[contract name](/latest/options/guides/instruments-and-symbols). Each contract publishes:

| Field                                        | Description                                                                                     |
| -------------------------------------------- | ----------------------------------------------------------------------------------------------- |
| `markPrice` / `bidPrice` / `askPrice`        | Mark, best-bid, and best-ask prices                                                             |
| `markIV` / `bidIV` / `askIV`                 | Implied volatility per side (see [Pricing & Greeks](/latest/options/guides/pricing-and-greeks)) |
| `delta` / `gamma` / `vega` / `theta` / `rho` | Greeks                                                                                          |
| `forward`                                    | Forward price                                                                                   |
| `openInterest`                               | Total open contracts                                                                            |
| `volume24h`                                  | 24-hour traded volume                                                                           |

Contracts are scoped by **underlying** and **expiration date**. Stream what's tradable live
on the [Contracts Stream](/latest/options/websocket-api/contracts-stream), or list them with
the [Get Expirations](/latest/options/rest-api/get-expirations) and
[Get Contracts](/latest/options/rest-api/get-contracts) REST endpoints.

Each expiry also carries an `expectedPriceMove`, with the `atmIV` and `indexPrice` behind it —
see [Expected Price Move](/latest/options/formulas/pricing-and-greeks#expected-price-move).

## Quotes

The [Quotes Stream](/latest/options/websocket-api/quotes-stream) delivers **top-of-book**
best bid/ask (with size and IV) for each contract — a lighter feed than full contracts when
you only need the current top of book.

## Order Book

The [Order Book Stream](/latest/options/websocket-api/order-book-stream) provides the full
book for a single contract as a snapshot followed by delta updates. Apply deltas in order to
maintain a local book.

## Trades

The [Public Trades Stream](/latest/options/websocket-api/public-trades-stream) provides
real-time executions for a contract, and a snapshot of recent trades on request.

## Prices Used Across the API

| Price            | Where it comes from                    | Used for                                                          |
| ---------------- | -------------------------------------- | ----------------------------------------------------------------- |
| Index price      | Underlying spot index                  | Margin, settlement reference                                      |
| Mark price       | Black-76 fair value from mark IV       | Margin, unrealized PnL                                            |
| Forward price    | Expected underlying price at expiry    | Black-76 pricing input                                            |
| Settlement price | Underlying price at expiry (08:00 UTC) | [Settlement](/latest/options/guides/settlement-and-expiry) payoff |