How to read $[G:N]$? For a group $G$ and a normal subgroup $N$ of $G$, the quotient group of $N$ in $G$ is written $G/N$.
I could find from this link how to read $G/N$ ("$G$ modulo $N$" or "$G$ mod $N$"), but I couldn't find $[G:N]$ (the order of $G/N$).
How to read $[G:N]$? 

"The index of $N$ in $G$". That notation does not require normality (it can be seen as the number of left cosets of $N$, for instance), and the number it represents might of course be infinity.
