> For the complete documentation index, see [llms.txt](https://elysia.gitbook.io/elysia-perps/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://elysia.gitbook.io/elysia-perps/mechanics/mark-and-oracle.md).

# Mark & Oracle Price

Two reference prices run beneath every market. They are separate from the **last traded price** you see fill on the book, and each has a specific job.

![Index, mark, and last price on one chart, each used for a different job](https://1949530945-files.gitbook.io/~/files/v0/b/gitbook-x-prod.appspot.com/o/spaces%2FDKUIMv3mUd02iALaez0P%2Fuploads%2Fgit-blob-60ec1074ac24fefe26d68b76c5ad5eac4a4f308b%2Fthree-prices.png?alt=media)

> The price sources and formulas below are the current settings on Elysia Perps' live markets. As with every parameter, the module's direction is for a market to configure its own price sources.

## Index price

The **index price** is the underlying price, taken as a **median of several venues**. For BTC-PERP-EL, it is the spot price across major spot venues (Binance, Bybit, and OKX). For US500-PERP-EL, the index price is the median of the SPY perpetual index prices published by Binance (USDT-M), Bitget (USDT-M), and Lighter. Using several sources and taking the median means no single venue can move it on its own. The index price is what **funding** is measured against. See [Funding](/elysia-perps/mechanics/funding.md).

## Mark price

The **mark price** is the *fair* price the protocol uses for anything where a manipulated number would be dangerous: **margin, liquidation, and unrealized PnL display**. Using the last traded price for these would let a single thin-book trade trigger unfair liquidations, so the mark price is built to resist that.

It is a **median of three independent inputs**:

* an **impact price** derived from real order-book depth on this market,
* a **smoothed premium price** built from the index plus a time-averaged premium, and
* the **perpetual mark price** of major external venues.

Taking the median of three sources means an outlier in any one of them is ignored.

## Why mark price can differ from the last trade

Because the mark price blends several sources, it can sit slightly away from the last price that printed on the book. That is expected: it protects you from liquidations driven by a momentary wick, and it is why your unrealized PnL is quoted against the mark rather than the last trade.

## Price freshness

Sources are polled continuously. If a source stops responding, its last value is held briefly and then dropped from the calculation; the remaining sources keep the price running. The price is never left to a single stale source without flagging it.

On US500-PERP-EL, if all three sources stop updating for more than 120 seconds, the market halts: new orders and order changes are rejected, and the mark price holds its last value.

## Where prices are used

| Price                 | Used for                                           |
| --------------------- | -------------------------------------------------- |
| **Index price**       | Funding settlement                                 |
| **Mark price**        | Margin, liquidation, unrealized PnL display        |
| **Last traded price** | The price your orders actually fill at on the book |

## Where to go next

* How the mark price drives liquidation: [Liquidation](/elysia-perps/mechanics/liquidation.md).
* How the index price drives funding: [Funding](/elysia-perps/mechanics/funding.md).