Muons are highly penetrating charged particles which can travel large distances through matter without interacting, compared to other charged particles. To identify muons, particle detectors are placed outside the main body of the detector, sandwiched between several layers of iron (also used as the flux return iron for the superconducting coil) which stop most other kinds of particles. The muon detectors are "plastic streamer counters", which operate somewhat like the drift chambers in the central detector region. The layout of the muon tracking chambers is shown here in cross section:
The actual position of a hit on one of the wires is determined from the ratio of the pulse heights read out at the two ends of each line.