Explanation of notation for face and degeneracy

Question

The (co)face are historically denoted by $d$, while the (co)degeneracy maps are denoted by $s$. Why is this, instead of the obvious $f$ and $d$? In the simplicial category $\Delta$, you also realize the face maps by "skipping" a number, and so even if you wanted to use the letters $s,d$, why not $s$ for skip and $d$ for degeneracy? I imagine they arose from abbreviating the terms in another language, but would like to know the actual reason.