# Understanding OP\_DEPTH
**OP\_DEPTH** examines the entire stack and pushes a number representing how many items currently exist. This count includes all items on the main stack at the moment of execution.
### Basic Operation
Consider a stack with three items:
```
Stack:
OP_DEPTH
Stack: 3
```
The number **3** appears on top because there were three items present. The original items remain untouched—OP\_DEPTH only adds information, it doesn't remove anything.
**Important note:** OP\_DEPTH counts the stack *before* adding its own output. It measures what was there, then adds the measurement result.
#### Common Use Pattern: Conditional Branching
The most powerful application of OP\_DEPTH is **adaptive validation**—scripts that change behavior based on how many inputs they receive.
**Example: Flexible authentication**
```
OP_DEPTH OP_1 OP_EQUAL OP_IF
OP_CHECKSIG
OP_ELSE
OP_DUP OP_HASH160 OP_EQUALVERIFY OP_CHECKSIG
OP_ENDIF
```
**How this works:**
1. **Check depth:** OP\_DEPTH counts stack items
2. **Compare to 1:** OP\_1 OP\_EQUAL checks if count equals 1
3. **Branch A (if 1 item):** Assume it's a signature, verify against stored public key
4. **Branch B (if >1 item):** Assume public key + signature, verify against stored hash
**Why this matters:**
* **Compact representation:** One script handles two payment patterns
* **Backward compatibility:** Can accept legacy signature-only inputs
* **Efficiency:** No need for separate script versions
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