To get to different coordinate systems I think you first need to define a 'container', containing them. Then you need a 'system' defining limits as I think. 'c' is a very peculiar limit, as it is valid (locally measured) for/in all uniformly moving frames of reference, no matter what speed you measure something else to have relative yourself. That will give you a multitude of coordinate system, as each observer strictly defined can be assumed to have his own unique coordinate system, depending on mass (energy), acceleration deceleration, and relative motion. That doesn't mean that they aren't applicable, or have a relevance. The problem for me is to to see when they have it

and I also suspect that Pete has a much better training in defining for what situations they are valid, than I ever will.

=

I should mention a arrow for it too, and, to me, that arrow is locally equivalent to 'c', meaning that is 'constantly there' for you. So whenever I talk about 'c' nowadays I'm also thinking of it as a definition of a arrows 'local and constant property'... Da*n it, I should also mention mass, but I won't

.. Or i did, maybe?