  Assessing the origin of Pluto and Triton has profound implications for the
bigger picture of Solar System formation and evolution. In such a context, this
chapter reviews our current knowledge of the formation conditions of Pluto and
Triton's constitutive building blocks in the protosolar nebula, which can be
derived from their known or estimated volatile contents. Assuming that the
ultravolatiles carbon monoxide and dinitrogen detected in Pluto and Triton are
primordial, the presence of these molecules suggest that the two bodies
accreted material originating from the vicinity of the carbon monoxide and
dinitrogen icelines. Dinitrogen--rich and water--poor comets such as comet
C/2016 R2 (PanSTARRS) obviously present a compositional link with Pluto and
Triton, indicating that their building blocks formed in nearby regions of the
protosolar nebula, despite of the variation of the water abundance among those
bodies. Also, the assumption of Triton's growth in Neptune's circumplanetary
disk requires that its building blocks formed at earlier epochs in the
protosolar nebula, to remain consistent with its estimated composition.
