Is voltage a delta? Can it always be treated as a potential difference from a reference point?

I've always considered voltage to be absolute, i.e. something that is there or is not there. However, the more I think about it, it seems to be more like a delta.

For example, let's say we have a supply that lists its pins as (A) -50V and (B) 0V. If we treat pin (A) as "ground", i.e. as if it were 0V, can we treat pin (B) as +50V?

Another example might be that pin (A) is +10V, and pin B is +25V, so the potential difference is +15V. Can we treat this the same way as if it were 0V and +15V?

Answer

You are always using/measuring the difference in potential between two points.

There is no absolute zero in voltage (like there is with temperature), although it is common practice to define earth as 0V. This is not absolutely necessary, you can use any potential as reference.