I'd never thought about why telnet uses ^] as its default escape sequence, but I just formed a theory while looking at an ASCII chart.<p>The ASCII ESC character is 1B, or ^[ (which many vi users know if they have a keyboard without Esc, or prefer to use ^[ for ergonomic reasons).<p>Presumably telnet has an important requirement that you be able to use vi -- or other software where you'll need to send ESC -- normally on the remote machine. If you're on a system where you're sending ^[ for ESC, what's the natural similar-but-different alternative for a meta-escape? Presumably ^], which is right next to it on your keyboard and conceptually related. That's the GS ("group separator") character, which isn't commonly used for anything on Unix.