What tool is primarily used for ensuring a structure is level?

The tool primarily used for ensuring a structure is level is the spirit level. A spirit level is designed to indicate whether a surface is horizontal (level) or vertical (plumb) by showing the position of a bubble within a liquid-filled vial. When the bubble is centered between the lines on the vial, it means the surface is perfectly level. This is crucial in construction, as buildings need to be level for aesthetic, functional, and safety reasons.

While the square, measuring tape, and plumb tool are essential for various laying out and measuring tasks, they serve different purposes. The square is used to ensure right angles and squareness in construction, the measuring tape is for determining lengths or distances, and the plumb tool, which is also important, is primarily used to check vertical alignment rather than horizontal levelness. Therefore, the spirit level stands out as the most effective tool for confirming that a structure is level.