# What Are Stack Data Queries?

Stack data query opcodes differ from other Bitcoin Script operations because they **provide information** **rather than transform data**. Instead of modifying stack contents, they measure and report on stack characteristics.

Think of stack queries as diagnostic tools. Just as a doctor checks vital signs before treatment, scripts use stack queries to assess conditions before proceeding with validation or computation.

**Two fundamental queries:**

| Word      | Input | Output           | Description                                  |
| --------- | ----- | ---------------- | -------------------------------------------- |
| OP\_DEPTH | items | items, qty items | Counts stack items and pushes the count      |
| OP\_SIZE  | item  | item, item size  | Measures top item length without removing it |

**Key characteristic:** Both opcodes are **non-destructive**—they leave the original stack contents intact while adding measurement information.

<figure><img src="/files/SOfXLyGr0KGwXARDckD5" alt=""><figcaption></figcaption></figure>


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