  Calibrations are a possible tool to validate the minimality of a certain
candidate. They have been introduced in the context of minimal surfaces and
adapted to the case of Steiner problem in several variants. Our goal is to
compare the different notions of calibrations for the Steiner Problem and for
planar minimal partitions. The paper is then complemented with remarks on the
convexification of the problem, on non-existence of calibrations and on
calibrations in families.
