[en] A method is described which defines an accurate reference system in stacks of nuclear emulsions, are required by present-day hybrid experiments. It is based on a new reference grid and a versatile X-ray marking set-up; both devices have been developed and are in use at CERN. The 240 mm x 120 mm grid is drawn with lines less than 20 μm wide. It has a basic 1 mm² mesh with 200 micron subdivisions. Coordinates are displayed in each square millimetre by sets of dots in binary code. The X-ray set-up includes a high-accuracy collimator which makes it possible to obtain very sharp lines down to 15 μm wide over a length of 100 mm. Different marking schemes can easily be implemented by means of long-range micrometric translating and rotating stages. (orig.)