  Initial state radiation (ISR) in deep inelastic scattering of charged leptons
on nucleons can lead to substantial shifts in the reconstruction of $Q^2$ and
the scaling variables $x$ and $y$. By measuring at least three out of the four
variables: energy and production angle of the lepton and the hadron system, the
energy of the ISR photon emitted under zero degrees can be determined. The ISR
effect is illustrated for experimentation at HERA with different methods of
kinematic reconstruction.
