A coordinate system with two intersecting, orthogonal axes one of which (the x-axis) lies in the galactic plane and the other (the z-axis) is perpendicular thereto. One coordinate (called galactic longitude) is the angle from the plane of the two axes to the plane containing the z-axis and the point in question; the second coordinate (called galactic latitude) is the complement of the angle from the z-axis to the line between the point in question and the point of intersection of the two axes. Two such systems have been in general use. The first, established by Ohlsson's conversion tables of 1932, placed the pole of the system at 12h 40m right ascension and +28^o declination. The origin for galactic longitude was the ascending node of the intersection (the x-axis) of the galactic plane with the plane of the celestial equator. Coordinates in this system are labelled with a subscript I. In 1958, the International Astronomical Union adopted a system in which the galactic pole is at 12h 49m right ascension, +27. 4^o declination, and the origin of galactic longitudes is at a position angle of 123^o with respect to the pole of the equator. The equator and equinox of the old system are those of 1900. 0; those of the new system are of 1950. 0 (and are labelled with the subscript II). Galactic longitudes in the new system are approximately 32^o greater that the longitudes in the old system.