What is the first rule to follow before adjusting the tension on a saw chain?

Turning off the saw is critical before adjusting the tension on a saw chain for safety reasons. A running chainsaw poses a significant risk of injury from the moving chain, which could cause accidental cuts or other serious injuries if one were to come into contact with it. Ensuring that the saw is powered down eliminates this danger, allowing for safe handling and adjustments without the risk of unintended operation. This precaution is a fundamental safety measure that must be observed whenever performing maintenance or adjustments on power tools like chainsaws.

While ensuring the chain is clean and wearing protective goggles are also important practices, they are secondary to the immediate concern of safety presented by operating machinery. Checking the fuel level, although a practical consideration, doesn't pertain directly to the task of adjusting the chain tension.