For the purpose of Cataloguing diffuse objects in a wide range of dimensions, two alternative proposals are presented.
Problems similar to those discussed in connection with the nomenclature of optical HII-regions may occur with any diffuse objects such as molecular clouds, large reflection nebulae, HI-regions, and, of course, radio HII-regions.
The radio HII-regions are most commonly designated by the galactic numbers “G”. The galactic longitude forms the first part of this number, the galactic latitude the second part, both parts being usually accurate to the first decimal digit and separated by the sign of latitude. This system (also used sometimes for optical objects) was introduced by Mezger and Henderson (1967) in a survey at 5 GHz, with a half power beam width of the telescope of about 6'. The angular dimensions of their objects were in the range from 3 to 11'.