How to designate a coordinate system Imagine I have a coordinate system where the coordinates are $a$ and $b$ How to designate this system? $a-b$ $ab$ $a,b$ $aOb$ (where $O$ stands for origin). Is there an accepted standard? I have never seen one.

No accepted standard, but most I've seen have been $ab$, that is, the two axis names without a space. A close second would be $a-b$, with a hyphen. I usually see that when the axis names are words, like "voltage-current graph".