The horn / reflector system is designed to form a parallel beam of positively charged pions / kaons (the parent particles of the neutrinos) from the CNGS target over a large range of production angles and energies. At the same time, negatively charged particles (the parents of unwanted neutrino species) are defocused by the system. A set of typical tracks as generated in a Monte-Carlo simulation is shown below. Inserted in the right hind side of these figures is the radial distribution of these particles at the entrance to the decay tunnel, 100 metres after the target.