  The impressive power-law decay of the energy spectrum of cosmic rays over
more than thirty orders of magnitude in intensity and for energies ranging over
eleven decades between $\simeq 10^9 $eV and $\simeq 10^{20} $eV is actually
dotted with small irregularities. These irregularities are highly valuable for
uncovering and understanding the modes of production and propagation of cosmic
rays. They manifest themselves through changes in the spectral index
characterising the observed power laws. One of these irregularities, known as
the \textit{ankle}, is subject to conflicting interpretations for many years.
If contemporary observations characterising it have shed new lights, they are
still far from being able to deliver all the story. The purpose of this
contribution is to give an overview of the physics of cosmic rays in the energy
range where the transition between Galactic and extragalactic cosmic rays is
expected to occur, and to deliver several lines of thought about the origin of
the ankle.
