Final answer:
A jack should never be used without a jack stand underneath it. The jack stand serves as a fail-safe in case the jack fails, providing essential safety when working under a vehicle.
Step-by-step explanation:
You should never use a jack without a jack stand underneath it. A jack stand is a device used to hold a vehicle up safely and securely while it is being worked on. Using a jack alone is not safe because if the hydraulic system fails, the vehicle may collapse onto the user. By adding a jack stand underneath, you have a fail-safe in case the jack gives out. This is a standard safety procedure in fields like auto repair, vehicle maintenance, and similar operations where a jack may be used.
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