The trajectories of charged particles in a tracking detector are straight lines in the absence of a magnetic field. They become helices in the presence of a magnetic field, where the motion in the plane orthogonal to the magnetic field is circular and that parallel to the magnetic field is linear. The magnitude of the momentum is proportional to the bending radius. Since tracking detectors provide rather precise trajectory position measurements, also the momentum components are precisely measured. Precise charged-particle momenta are important kinematic observables in high-energy physics experiments since they play a crucial role, for example, in determining invariant masses of decaying particles, missing momenta and the jet energy.