  In view of recent developments attention is directed again at two aspects of
the well known 'knee' in the cosmic ray energy spectrum at 3 PeV: the mass of
the predominant particles at this energy and their source. It is inevitable in
a subject such as this that ideas - and conclusions - evolve. Earlier, we had
used a particular acceleration model and the nature of the local ISM to infer
that the particles are mainly oxygen nuclei. Initially no specific source was
identified. More recently, however, we have specified the Monogem Ring
supernova remnant (SNR) as the likely source; this is at just the right
distance and age and the energies are reasonable. Concerning the mass
composition at the knee, a quantity more difficult to determine, recent direct
measurements, which extend to higher energies than hitherto, show a likely
flattening in the spectrum above 10^4 GeV/nucleon for He-nuclei, a flattening
which, if extrapolated to higher energies, would meet the measured spectrum in
the knee region. The other nuclei do not show this feature. He-nuclei in the
knee region would also be marginally consistent with KASCADE EAS data, although
there are serious problems with EAS mass estimates in that experiment.
Concerning the acceleration, recent models applied to the Monogem Ring SNR
allow a satisfactory explanation in terms of either oxygen or helium, but with
the latter being a distinct possibility and perhaps more likely.
