It has been estimated that up to 10% of Kuiper-belt objects may be gravitationally bound to form stable two-body configurations. All known examples of these binaries orbiting the Sun beyond Neptune are unusual for their mass ratios of order unity and large elliptic mutual orbits. Based on four-body (Sun, binary, intruder) simulations in the Hill approximation, a common dynamical origin for compositional and orbital properties of transneptunian binaries is proposed. The chaos-assisted capture mechanism and possible energy loss effects involved in post-capture evolution will be discussed.