Page "Classical Kuiper belt object" Paragraph 24

Another difference between the low-inclination ( cold ) and high-inclination ( hot ) classical objects is the observed number of binary objects.

Binaries are quite common on low-inclination orbits and are typically similar-brightness systems.

Binaries are less common on high-inclination orbits and their components typically differ in brightness.

This correlation, together with the differences in colour, support further the suggestion that the currently observed classical objects belong to at least two different overlapping populations, with different physical properties and orbital history.