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Prior to the establishment of a fundamental catalogue of positions, it is necessary to define the reference system which postulates the basic principles to be followed in order to arrive at the star coordinates of the fundamental catalogue. Usually, these principles include the concept of an inertial system defined such that in the framework of Newtonian mechanics the differential equations of motion can be written without terms involving linear and rotational accelerations. These principles also comprise the models and constants employed for deriving the coordinates. In the original terminology a fundamental catalogue of positions consists of star places, which are inferred from absolute observations of the equatorial coordinates, right ascension and declination. These observations are made with one single instrument, a transit or meridian circle, for instance, and without reference to previously determined positions of stars. The right ascensions of stars in such an instrumental or individual catalogue of absolute observations are obtained by comparing the transits of stars across the local meridian with those of objects of the solar system, predominantly the Sun, whose ephemerides are a realization of the celestial reference system that is defined by the solar system. A sidereal clock is required for recording these transit times. It is one of the most delicate tasks of meridian circle astrometry to determine the clock corrections in order to provide the correct sidereal time (Sect. 2.3.2). As an alternative, the so-called equatorial clock-stars may serve as a s/b ‘sidereal clock’ (Sect. 1.7). The determination of absolute declinations of stars is made from observations at upper and/or lower culminations at transit time, but use is made of celestial pole determinations from observations of circumpolar stars at upper and lower culminations. Often, these observations are supplemented by observations of solar system objects to determine the position of the equator from their declinations in relation to the ephemerides. These types of catalogues are usually designated individual absolute observation catalogues to distinguish them from the more customary present-day definition of fundamental catalogues given below.