  The MUon Scattering Experiment, MUSE, at the Paul Scherrer Institute,
Switzerland, investigates the proton charge radius puzzle, lepton universality,
and two-photon exchange, via simultaneous measurements of elastic muon-proton
and electron-proton scattering. The experiment uses the PiM1 secondary beam
channel, which was designed for high precision pion scattering measurements. We
review the properties of the beam line established for pions. We discuss the
production processes that generate the electron and muon beams, and the
simulations of these processes. Simulations of the $\pi$/$\mu$/$e$ beams
through the channel using TURTLE and G4beamline are compared. The G4beamline
simulation is then compared to several experimental measurements of the
channel, including the momentum dispersion at the IFP and target, the shape of
the beam spot at the target, and timing measurements that allow the beam
momenta to be determined. We conclude that the PiM1 channel can be used for
high precision $\pi$, $\mu$, and $e$ scattering.
