Is there a standard for headstone orientation?

[Black]

I was passing a cemetery today and got to wondering how the headstones could be allowed to be skewed all different ways.
I imagine they are oriented such so that if you stand directly in front of them they appear vertically level? Or are they level initially despite the slope they're on, and I'm just seeing a lot of settling?
I don't need the answer for a few hours, so take your time.