How do I make a setblock or a fill clock?

Often, when asking about command block contraptions, people suggest using a "setblock" or "fill" clock?

What are those and how do I make them?

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How to Enter the Command

Open the Chat Window. The easiest way to run a command in Minecraft is within the chat window.

Type the Command. In this example, we will replace a block with birch planks using the following command: /setblock ~1 ~0 ~1 planks 2.

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