# Building Blocks: Constant Value Opcodes

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Before diving into complex operations, Bitcoin Script provides a shortcut for the most common values you'll need: **small integers from -1 to 16, plus an empty value**. Rather than requiring multiple bytes to represent these simple numbers, Bitcoin includes dedicated **constant value opcodes** that push these values onto the stack with a single byte.

#### The Complete Set of Constants

Bitcoin Script includes **18 constant value opcodes** divided into three categories:

| **Opcode**                                              | **Pushes Value**   | **Description**                               |
| ------------------------------------------------------- | ------------------ | --------------------------------------------- |
| **OP\_0** (also called **OP\_FALSE**)                   | Empty array (null) | Pushes an empty byte array onto the stack     |
| **OP\_1NEGATE**                                         | -1                 | Pushes the number negative one onto the stack |
| **OP\_1** (also called **OP\_TRUE**) through **OP\_16** | 1 through 16       | Pushes integers 1 through 16 onto the stack   |

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These opcodes are remarkably efficient. Instead of using multiple bytes to represent "push 1 byte containing value 3," you simply use **OP\_3**—one byte that accomplishes the entire operation.

#### Why These Specific Values?

You might wonder why Bitcoin Script includes constants for -1 through 16 but not, say, -5 or 20. The answer lies in **practical usage patterns**:

* **OP\_0 / OP\_FALSE**: Essential for boolean logic and empty placeholders
* **OP\_1 / OP\_TRUE**: Common in boolean checks and counter initialization
* **OP\_1NEGATE (-1)**: Used in certain mathematical operations and protocol patterns
* **OP\_2 through OP\_16**: Frequently appear in multisignature scripts, counter operations, and threshold checks

The most common example is **multisignature scripts**, which specify how many signatures are required (M) out of how many public keys (N). For a 2-of-3 multisig, the script uses **OP\_2** and **OP\_3** directly rather than pushing these values as data.

#### Practical Example: 2-of-3 Multisignature

Here's how constant values appear in a real multisignature script:

```
OP_2 <pubkey_1> <pubkey_2> <pubkey_3> OP_3 OP_CHECKMULTISIG
```

**Breaking down the constants:**

* **OP\_2**: Pushes the value 2 onto the stack (number of required signatures)
* **OP\_3**: Pushes the value 3 onto the stack (total number of public keys)

The **OP\_CHECKMULTISIG** opcode reads these values to understand "verify that 2 out of these 3 public keys have provided valid signatures." Without constant value opcodes, you'd need to encode each number as a data item, adding unnecessary bytes to every multisignature transaction.

#### Key Characteristics

Understanding how constant opcodes behave helps you recognize them in scripts:

1. **Single-byte efficiency**: Each constant requires only one byte in the script
2. **No following data**: Unlike pushdata opcodes, constants don't read subsequent bytes
3. **Immediate values**: The value is pushed instantly during execution
4. **Boolean aliases**: OP\_0/OP\_FALSE and OP\_1/OP\_TRUE can be used interchangeably
5. **Limited range**: Only -1 through 16 are available; other numbers require pushdata


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