A spirit level is a tool that determines whether a surface is perfectly horizontal or vertical by showing a bubble's position within a sealed vial filled with a colored fluid, often a mixture of alcohol or ether and water. The bubble always floats to the highest point, and when it is centered between two markings on the vial, the object is level. This simple, inexpensive device is a fundamental instrument in construction, carpentry, and surveying to ensure accurate installation and alignment.