A level tool, also known as a spirit level or a bubble level, helps ensure lines are entirely straight. They've been used for centuries and while there are many different types of level tools, they all serve the same basic function.
A level tool is a small, flat device that holds a slightly curved glass vial. Some older models may come with a second glass vial.
The glass vial is filled almost entirely with ethanol, leading to its nickname of "spirit level," and is often yellow colored. Ethanol is used due to its low viscosity, as it allows for the air bubble to move easily and settle quickly with the least amount of interference.
There is also a blue color that is sometimes used, which comes with a tube that is shaped slightly different from a normal level tool. Both are effective, but the blue-filled level tool can be more accurate and some find them easier to read.
The liquid in a level tool is never filled entirely; there is always room left for a single air bubble. When placed upon a flat surface, the bubble will rise and stay in the center of the glass tube, often found between two lines marked on the outside of the vial.