Which tool is used to determine a horizontal plane?

A Wasserwaage, commonly known as a spirit level, is the tool specifically designed to determine a horizontal plane. It consists of a sealed tube containing a liquid with a bubble inside. When the bubble is centered between two marked lines, it indicates that the surface being assessed is level with the horizontal plane. This makes it an essential instrument for tasks that require precise leveling, such as construction and carpentry, ensuring that surfaces like floors, shelves, and frames are aligned correctly.

The other tools listed serve different purposes: a Lot, or plumb line, measures vertical alignment; a Gliedermaßstab, or folding rule, is primarily used for measuring length; and a Rasterlehre, or grid ruler, is used for scaling purposes in design and drafting. Each of these tools plays a specific role in measuring or aligning, but none is specifically intended for assessing horizontal planes like the Wasserwaage.