  In Newtonian space-time there exist four, and only four, causal classes of
frames. Natural frames allow to extend this result to coordinate systems, so
that coordinate systems may be also locally classified in four causal classes.
These causal classes admit simple geometric descriptions and physical
interpretations. For example, one can generate representatives of the four
causal classes by means of the {\em linear synchronization group}. Of
particular interest is the {\em local Solar time synchronization}, which
reveals the limits of the frequent use of the concept of `causally oriented
oordinate', such as that of `time-like coordinate'. Classical {\em positioning
systems}, based in sound or light signals, are, by themselves, interesting
examples of location systems, i.e. of physically constructible coordinate
systems. They show that one can locate events in Newtonian space-time {\em
without} any use of the concept of synchronization. In fact, the coordinate
systems associated to positioning systems, belong to all the classes but the
standard one, i.e. the one based in the simultaneity synchronization. The
relativistic analogs of these examples, emphasize the contrast between the four
Newtonian and the one hundred and ninety nine Lorentzian causal classes of
frames of classical and relativistic space-times, respectively.
