The system of coordinates that is used in an inertial reference frame, normally in the special theory of relativity. The three space coordinates are usually Cartesian coordinates (x, y, z), and the time coordinate is the time as measured by an observer at rest in the coordinate system. In astrometry, an inertial coordinate system is a reference frame formed by assigning coordinates to specific observable objects, such as the positions and proper motions of stars in a fundamental catalogue.