Specifically I'm reading Dirac's General Relativity and he says essentially:
$$ \delta Q = \frac{\partial Q}{\partial x^\mu} \delta x^\mu $$
But what's the difference between this and:
$$ dQ = \frac{\partial Q}{\partial x^\mu} d x^\mu $$
From what I understand this is some kind of variational calculus thing, but I don't explicitly know, it seems completely identical.
Here's the context in which I am reading it from if it helps at all:
