# How to adjust for the confounder of a confounder and how to call the confounder of a confounder within treatment effect estimation?

How do we adjust for the confounder of a confounder in order to compute unbiased estimates of the treatment effect of $$A$$ on $$D$$? See the causal graph (DAG) below:

What do we call the confounder $$C$$ (of a confounder $$B$$) - does we give such variables/confounders any special name?

The answer here is pretty straight-forward: you simply condition on both $$C$$ and $$B.$$ Neither $$C$$ nor $$B$$ is part of a collider, so you're not opening up new paths by doing so, and this conditioning closes all backdoor paths from $$A$$ to $$D,$$ enabling you to get the unbiased effect of $$A$$ on $$D.$$
I'm not aware of any special name for the $$C$$ relative to $$B$$ except "parent". I'm also not sure I would call $$C$$ a "confounder of a confounder". A confounder is a variable that sets up a backdoor path from the cause in which you're interested to the effect in which you're interested. But you're not interested in $$B$$ as a cause of anything, so it strikes me as moot in this situation. I would just call $$C$$ the parent of $$B$$ and leave it at that.