When using a line level, it is very important to check if it is level first before using it. The top part of the line should always be flat against the surface of the object being measured. To check if your line is level, look at the bubble on the top part of the line level and see if it is exactly centered on the bubble level. If it is not centered on the bubble level, it means that your line is not level and needs to be straightened before using it. If your line is not straightened before using it, it might cause your measurements to be inaccurate and might even cause accidents if it falls down or touches another object while measuring.