The kinematic variables of the particles transporter by GEANT
are always referred to the so-called
MAster Reference System ( MARS). This system
is implicitly defined as the local reference system of the first
volume defined, which contains all the others. This is a Cartesian
coordinate system with axis
where
.
If the axes are labelled (X,Y,Z), then the point P
is represented in fig 
.
Figure: GEANT reference system
Tracking is performed in the MARS and the input position for user routines such as the magnetic field routine is given in this system.