line-number-mode

line-number-mode is an interactive compiled Lisp function in `simple.el'.

It is bound to <menu-bar> <options> <showhide> <line-number-mode>.

(line-number-mode &optional ARG)

Toggle line number display in the mode line (Line Number mode).
With a prefix argument ARG, enable Line Number mode if ARG is
positive, and disable it otherwise. If called from Lisp, enable
the mode if ARG is omitted or nil.

Line numbers do not appear for very large buffers and buffers
with very long lines; see variables `line-number-display-limit'
and `line-number-display-limit-width'.