A beam of high-energetic (190 GeV) charged particles (pions) impinges (coming from the lower left corner) on a fixed target of transversely spin-polarized protons. The red dots indicate hits in the various COMPASS tracking detectors registered in coincident mode. The true hit information is determined by looking up information in alignment and calibration data bases. A tracking algorithm tries to find the best possible fit of these about 200 hits and reconstructs the trajectory of each charged particle. From the bending radius in the field of two strong dipole magnets, the momentum of each charged particle is determined, while other detectors allow to identify different particle species.