Cosmic rays have been observed with energies from 109 eV to over 1020 eV. Over this range, the "flux" of cosmic rays (the number of arriving particles per unit area, per solid angle, per unit time) appears to follow a single power law ~E-3. The variation of the flux with energy is referred to as the "Energy Spectrum". This spectrum is shown in the figure to the right. Remarkably, it appears to be a smooth curve over 10 decades of energy with only a few noticeable structures. The most prominent of these are small, abrupt changes in the local spectral index (the power exponent ~3) just above 1015 eV and again just above 1018 eV. Within the Cosmic Ray field, these features are usually referred to as the "knee" and the "ankle", respectively.