What tool indicates whether a surface is true by using a liquid-filled tube?

The tool that indicates whether a surface is true by using a liquid-filled tube is known as a level. A level typically contains a sealed glass or plastic tube filled with a liquid, usually colored for visibility, with a small air bubble inside. When the level is placed on a surface, the bubble will rise to the highest point, allowing the user to determine if that surface is perfectly horizontal (level) or perfectly vertical (plumb).

This functionality is essential in many construction, carpentry, and other trades to ensure objects are aligned accurately. The use of liquid in the tube helps in providing a precise reading, as the bubble will always rise to the highest point due to gravity, effectively allowing users to gauge whether they have achieved the desired alignment accurately.