Spirit level

A spirit level, bubble level, or simply a level, is an instrument designed to indicate whether a surface is horizontal (level) or vertical (plumb). It is called a "spirit level" because the liquid inside its vial is commonly alcohol, or "spirit". The name refers to this alcohol-based solution, which, along with an air bubble, indicates whether a surface is level or plumb.

Alcohol was historically preferred over water because it has a wider temperature range, won't freeze, and provides less friction for the bubble, ensuring greater accuracy and longevity of the tool.
Two basic designs exist: tubular and bull's eye .
Different types of spirit levels may be used by carpenters, stonemasons, bricklayers, other building trades workers, surveyors, millwrights and other metalworkers, and in some photographic and videographic work.

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Source: Wikipedia · Image: Wikimedia Commons · Public domain