  The cylindrical drift chamber is the most innovative part of the MEG~II
detector, the upgraded version of the MEG experiment. The MEG~II chamber
differs from the MEG one because it is a single volume cylindrical structure,
instead of a segmented one, chosen to improve its resolutions and efficiency in
detecting low energy positrons from muon decays at rest. In this paper, we show
the characteristics and performances of this fundamental part of the MEG~II
apparatus and we discuss the impact of its higher resolution and efficiency on
the sensitivity of the MEG~II experiment. Because of its innovative structure
and high quality resolution and efficiency the MEG~II cylindrical drift chamber
will be a cornerstone in the development of an ideal tracking detector for
future positron-electron collider machines.
