A field entity comprising the electric field E and the magnetic field (or magnetic induction) B. All electric and magnetic fields are electromagnetic fields because the decomposition of these fields is not unique. This is evident from the Lorentz force (see magnetic induction) on a charge, which depends on its velocity in a given inertial coordinate system. In another inertial coordinate system velocity, and hence the Lorentz force, is different. But ultimately it is only by means of the Lorentz force that fields are observed. Thus, two observers moving with a constant velocity relative to each other will not agree on whether a particular field is solely electric or solely magnetic.