A vial is what is read when using a level. It’s a tube of glass filled with liquid that has an air bubble inside. When the bubble fits between the two black lines, the surface is level.
To measure across more than one axis, use a circle vial. This type of vial is good for leveling tables and countertops. When the bubble fits into the center of the circle, the entire tabletop is level.
When choosing a level, look for one that is read easily. Make sure the bubble is clearly visible between the two black lines. If there’s too much space around the bubble, readings will not be as accurate.
Use a long level for larger spaces. A small level will not necessarily give accurate readings over the entirety of a long length.
Many levels have multiple vials to fit different needs. This contractor level has a vial for horizontal readings and another level to measure plumb.
This specialty level can assist with measuring to hang pictures or mirrors. It has a scale with a center locator to help make sure mounting locations are evenly spaced on the wall.
This level has a vial set at an angle to find a 45-degree reading.
A post level has three vials to help you make sure a fence post is plumb.