Variable: left-margin
left-margin is a customizable and buffer-local variable defined in
buffer.c.
Documentation
Column for the default indent-line-function to indent to.
Linefeed indents to this column in Fundamental mode.
Probably introduced at or before Emacs version 1.4.
Source Code
// Defined in /usr/src/emacs/src/buffer.c
DEFVAR_PER_BUFFER ("left-margin", &BVAR (current_buffer, left_margin),
Qintegerp,
doc: /* Column for the default `indent-line-function' to indent to.
Linefeed indents to this column in Fundamental mode. */);