The clinometer uses a level bubble to determine the local horizontal and then measures the angle (or slope) relative to that local horizon. A special version is the plumbing level that is essentially a clinometer set to 90 degrees allowing it to measure exactly up or down. This is a usefull feature when building for example a tall building that needs to be straight up. Clinometers are widely used in the artillery for setting the vertical angle of a (machine)gun. They are also used in surveying as a fast method of determining vertical angles without a theodlite.