line-prefix

line-prefix is a variable defined in `C source code'.
Its value is

nil

• Automatically becomes buffer-local when set.

Documentation:
Prefix prepended to all non-continuation lines at display time.
The value may be a string, an image, or a stretch-glyph; it is
interpreted in the same way as the value of a `display' text property.

This variable is overridden by any `line-prefix' text or overlay
property.

To add a prefix to continuation lines, use `wrap-prefix'.