The mutual inclination between two planets is the angle between their orbital planes. Many compact systems with multiple close-in planets interior to the equivalent orbit of Venus are expected to have very low mutual inclinations, so the system (at least the close-in part) would be even flatter than the Solar System. Captured planets could be captured into any arbitrary angle to the rest of the system. As of 2016 there are only a few systems where mutual inclinations have actually been measured One example is the Upsilon Andromedae system: the planets c and d have a mutual inclination of about 30 degrees.