Why are open sets denoted $U$, $G$, and measurable sets $E$? Why are open sets usually denoted by $U$?
Is there a reference about this?
Sometimes open set uses the letter $G$, such as $G_{\delta} $ set.
I also wonder the meaning of $G$.
Additional question: Why do we use or who first used $E$ to denote a subset in measure theory?

For $G_\delta$ set and $F_\sigma$ set, each of this is from german word Gebiet and french word fermé  respectively.
