Figure 8.1 shows the FNAL accelerator, experimental areas, and neutrino beam line. Protons with an energy of 400 GeV, from the main ring of the FNAL proton synchrotron, strike an aluminum target and produce a shower of hadrons, most of which are pions and kaons. These pass through a double-horn focusing device, in which a toroidal magnetic field defocuses the positive hadrons and focuses the negative ones into a 350-meter-long evacuated decay pipe. Here some of the hadrons decay into neutrinos and leptons before reaching an iron and Earth shield which absorbs the rest. Although the hadrons can be absorbed quickly via the strong interaction, the muons which are produced along with the neutrinos lose energy much more slowly, mainly via ionization. Hence, the shield extends for 1000m in order to eliminate virtually all the muons. At the end of the shield the beam contains mostly ##{\bar \nu}_\mu## from the dominant decay modes of the positive hadrons in the decay pipe, with a slight contamination of ##\nu_\mu##, ##{\bar \nu}_e## and ##\nu_e##.