Abstract
1. We present a very fast derivation of Minkowski’s line element which is the geometric foundation not only of the special Lorentz transformation but of the whole Poincar’e group aka inhomogeneous Lorentz group [1]. The Euclidean line element is excluded using the wave equation and Einstein’s special principle of relativity (Einstein’s first postulate) [2]. For doing so, it is not necessary, 1. to assume the speed of light to be independent of the motion of the observer (Einstein’s second postulate [2], 2. to synchronize watches, 3. to interrelate the coordinate systems of the inertial frames involved; their relative velocity (and hence reciprocity [3]) even plays not any role in the derivation. 4. to make any assumption about the properties of the underlying coordinate transformation such as linearity, uniqueness etc