Polar rings and the 3D-shape of dark matter

Polar ring galaxies (PRG) are unique to give insight on the 3D-shape of dark matter haloes. Some caveats have prevented to draw clear conclusions in previous works. Also the formation mechanisms need to be well known. All available information on the topic is reviewed, and criteria are defined for an ideal PRG system, in the hope of removing the ambiguities and make progress in the domain.