The disc origin of the Milky Way bulge
  The Galactic bulge, that is the prominent out-of-plane over-density present
in the inner few kiloparsecs of the Galaxy, is a complex structure, as the
morphology, kinematics, chemistry and ages of its stars indicate. To understand
the nature of its main components -- those at [Fe/H] >~ -1 dex -- it is
necessary to make an inventory of the stellar populations of the Galactic
disc(s), and of their borders : the chemistry of the disc at the solar
vicinity, well known from detailed studies of stars over many years, is not
representative of the whole disc. This finding, together with the recent
revisions of the mass and sizes of the thin and thick discs, constitutes a
major step in understanding the bulge complexity. N-body models of a
boxy/peanut-shaped bulge formed from a thin disc through the intermediary of a
bar have been successful in interpreting a number of global properties of the
Galactic bulge, but they fail in reproducing the detailed chemo-kinematic
relations satisfied by its components and their morphology. It is only by
adding the thick disc to the picture that we can understand the nature of the
Galactic bulge.
