# The Alternate Stack

<figure><img src="/files/4Y8NG7jeuNUmcwKi4QMQ" alt=""><figcaption></figcaption></figure>

Bitcoin Script provides a **secondary stack**—the **alternate stack (altstack)**—for **temporary data storage**. This is particularly useful in **complex scripts** where you need to **"set aside" values temporarily** while processing other data.

| Word              | Input   | Output  | Description                                |
| ----------------- | ------- | ------- | ------------------------------------------ |
| `OP_TOALTSTACK`   | x1      | (alt)x1 | Moves top item to altstack                 |
| `OP_FROMALTSTACK` | (alt)x1 | x1      | Moves top altstack item back to main stack |

The altstack operates on the same **LIFO principle**—the **last item moved to it** is the **first retrieved**.

\[Diagram: Main stack and alternate stack side-by-side, showing items moving between them]

#### Practical Pattern: Temporary Storage

```
// Store a value for later x1 x2 x3 OP_TOALTSTACK // Main stack: x1 x2 // Altstack: x3 // Do operations with x1 and x2 OP_ADD // Main stack: (x1+x2) // Retrieve stored value OP_FROMALTSTACK // Main stack: (x1+x2) x3
```

#### Advanced Example: Multi-Path Verification

The altstack enables **elegant solutions** for scripts with **multiple execution paths** where you need to **track state across branches**:

```
// Store path identifier on altstack OP_1SUB OP_DUP OP_TOALTSTACK OP_NOTIF // Path 1: Multi-sig 2 <pubkey1> <pubkey2> 2 OP_CHECKMULTISIG OP_FROMALTSTACK OP_DROP OP_ELSE // Path 2: Single sig OP_FROMALTSTACK OP_1SUB OP_NOTIF <pubkey> OP_CHECKSIG OP_ELSE OP_FALSE OP_RETURN OP_ENDIF OP_ENDIF 
```

By storing the **path identifier** on the altstack, the script can check it later without **cluttering the main stack** during signature verification.


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