  In HEP experiments the description of the trajectory of a charged particle is
obtained from a fit to measurements in tracking detectors. The parametrization
of the trajectory has to account for bending in the magnetic field, energy loss
and multiple scattering in the detector material. General broken lines
implement a track model with proper description of multiple scattering leading
to linear equations with a special structure of the corresponding matrix
allowing for a fast solution with the computing time depending linearly on the
number of measurements. The calculation of the full covariance matrix along the
trajectory enables the application to track based alignment and calibration of
large detectors with global methods.
