# How is $\delta s$ different than $ds$? [duplicate]

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: