If you are working underneath or close to your car with the wheels off the ground, don't rely on a jack to keep it safely supported. A jack stand is a strong steel support that holds your car firmly in place with little or no chance of it slipping and falling on you. A hydraulic or manual jack is an imperfect holding device due to the fact that it goes up and down. If a hydraulic jack fails suddenly it will fall, causing injury or even death. A stand has four feet firmly on the ground and a solid point to hold the car up. They should even be used in pairs.