Why do `ctrl+[` and `ESC` both produce `^[`?

In bash, I press ctrl+v to start verbatim insert. In the verbatim mode, I press the Esc key and bash shows ^. I redirect it to file esc. Also in the verbatim mode, I press ctrl key with [ key, and bash shows ^[. I redirect it to file ctrl. Next, I compare the two files, and they are the same! $ echo '^[' > esc $ echo '^[' > ctrl $ diff esc ctrl $ Why do Ctrl+[ and Esc produce the same content? Is ^[ here [the C0 and C1 control codes ? If so, the wiki article says ^[ is Escape, so why is ctrl+[ also Escape? The root problem is that I want to check and create a key binding. (zsh)$ bindkey -L ... bindkey "^['" quote-line ... So do I need to type ESC+' or ctrl+[+'?