Abstract: This is a self-contained introduction to the subject. It can be read in itself and can be regarded as a rounded off little book. At the same time, it serves as Part I of the more comprehensive work described in the abstract of On the logical structure of relativity theories. It contains an intuitive introduction for this area. Further intuitive motivation etc. is in the introduction of Part V (Chapter 6) of the book On the logical structure of relativity theories.