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# Extending Chains

## Overview

Every Chain returned by [`Chain.from`](/docs/chains/create#chainfrom) includes `.extend()` for
deriving a new Chain
from an existing base.

The method merges overrides into the base. The returned Chain also includes `.extend()` for further
derivations.

```ts twoslash
import { Chain } from 'viem'

const mainnet = Chain.from({
  id: 1,
  name: 'Ethereum',
  nativeCurrency: { name: 'Ether', symbol: 'ETH', decimals: 18 },
  rpcUrls: { http: 'https://eth.merkle.io' },
})

const custom = mainnet.extend({
  rpcUrls: { http: 'https://my-rpc.example' },
})
```

## Recipes

### Create a Client for the Derived Chain

Pass the derived Chain to [`Client.create`](/docs/clients/create). The Client uses the overridden
fields and inherits the remaining fields from the base.

```ts twoslash
import { Chain, Client, http } from 'viem'

const mainnet = Chain.from({
  id: 1,
  name: 'Ethereum',
  nativeCurrency: { name: 'Ether', symbol: 'ETH', decimals: 18 },
  rpcUrls: { http: 'https://eth.merkle.io' },
})

const chain = mainnet.extend({
  rpcUrls: { http: 'https://my-rpc.example' },
})

const client = Client.create({
  chain, // [!code focus]
  transport: http(),
})
```

## `chain.extend`

Derives a new chain from a base, merging the overrides over it.

### Usage

```ts twoslash
import { Chain } from 'viem'

const mainnet = Chain.from({
  id: 1,
  name: 'Ethereum',
  nativeCurrency: { name: 'Ether', symbol: 'ETH', decimals: 18 },
  rpcUrls: { http: 'https://eth.merkle.io' },
})

const custom = mainnet.extend({
  rpcUrls: { http: 'https://my-rpc.example' },
})
```

### Parameters

#### overrides

* **Type:** `Partial<Chain.Chain>`

The fields to merge over the base chain.

```ts twoslash
import { Chain } from 'viem'

const mainnet = Chain.from({
  id: 1,
  name: 'Ethereum',
  nativeCurrency: { name: 'Ether', symbol: 'ETH', decimals: 18 },
  rpcUrls: { http: 'https://eth.merkle.io' },
})
// ---cut---
const custom = mainnet.extend({
  rpcUrls: { http: 'https://my-rpc.example' }, // [!code focus]
})
```

### Return Value

`Chain`

A new chain with the overrides applied, preserving the merged literal type and exposing its own `.extend()`.