  The knee in the all-particle energy spectrum is scrutinized with a
phenomenological model, named poly-gonato model, linking results from direct
and indirect measurements. For this purpose, recent results from direct and
indirect measurements of cosmic rays in the energy range from 10 GeV up to 1
EeV are examined. The energy spectra of individual elements, as obtained by
direct observations, are extrapolated to high energies using power laws and
compared to all-particle spectra from air shower measurements. A cut-off for
each element proportional to its charge Z is assumed. The model describes the
knee in the all-particle energy spectrum as a result of subsequent cut-offs for
individual elements, starting with the proton component at 4.5 PeV, and the
second change of the spectral index around 0.4 EeV as due to the end of stable
elements (Z=92). The mass composition, extrapolated from direct measurements to
high energies, using the poly-gonato model, is compatible with results from air
shower experiments measuring the electromagnetic, muonic, and hadronic
components. But it disagrees with the mass composition derived from X_max
measurements using Cerenkov and fluorescence light detectors.
