# Understanding Flow Control

**Flow control opcodes** allow Bitcoin scripts to make decisions during execution. These opcodes determine whether to execute or skip sections of code based on the values at the top of the stack. Flow controHash Time-Locked Contractsl is Bitcoin Script's primary mechanism for implementing **conditional logic**.

#### Available flow control opcodes:

* **OP\_IF** — Executes code block if top stack item is non-zero (true)
* **OP\_NOTIF** — Executes code block if top stack item is zero (false)
* **OP\_ELSE** — Provides alternative execution path
* **OP\_ENDIF** — Marks the end of a conditional block

#### How flow control works:

1. An **expression** evaluates and leaves a result on the stack
2. The flow control opcode (**OP\_IF** or **OP\_NOTIF**) examines the top stack item
3. Based on that value, the script either executes the code inside the conditional block or skips to **OP\_ELSE** (if present) or **OP\_ENDIF**
4. Script execution continues after the conditional block

#### Why this matters:

Flow control enables **multiple spending conditions** in a single output, creates **conditional payment paths** based on time or signatures, and implements **fallback mechanisms** when primary conditions aren't met. This powers advanced contracts like escrow, atomic swaps, and payment channels.


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