Kaons and pions are momentum and charge selected in a short transfer line by means of dipole and quadrupole magnets and are focused in a collimated beam into an instrumented decay tube. Large angle muons and positrons from kaon decays ( K + → → μ μ + ν ν μ μ , K + → → μ μ + π π 0 ν ν μ μ , K + → → e + π π 0 ν ν e ) are measured by detectors on the tunnel walls, while muons from pion decays ( π π + → → μ μ + ν ν μ μ ) are monitored after the hadron dump at the end of the tunnel. The decay region is kept short (40 m) in order to reduce the neutrino contamination from muon decays ( μ μ + → → e + ν ν e ν ν ¯ ¯ μ μ ).