On graphical displays, each Emacs window normally has narrow fringes (gutters/margins) on the left and right edges. The fringes are used to display symbols that provide information about the text in the window. You can type M-x fringe-mode to disable the fringes, or modify their width. This command affects fringes in all frames; to modify fringes on the selected frame only, use M-x set-fringe-style. You can make your changes to the fringes permanent by customizing the variable fringe-mode.

Out-of-the-box the most common use of the fringes is to indicate a continuation line. When one line of text is split into multiple screen lines, the left fringe shows a curving arrow for each screen line except the first, indicating that “this is not the real beginning”. The right fringe shows a curving arrow for each screen line except the last, indicating that “this is not the real end”. If the line’s direction is right-to-left, the meanings of the curving arrows in the fringes are swapped.

Third-party modes like flycheck and diff-hl also make use of the fringe to display valuable information there (e.g. lint and VC information).

By default both fringes have width 8 pixels, but we can easily adjust this:

;; make both fringes 4 pixels wide
(fringe-mode 4)

;; make the left fringe 4 pixels wide and the right disappear
(fringe-mode '(4 . 0))

;; restore the default sizes
(fringe-mode nil)

As mentioned before, you can also invoke this command interactively and determine the optimal fringe size for you, before making it permanent in your config. The options presented by the fring-mode command are defined in the fringe-styles list:

(defconst fringe-styles
  '(("default" . nil)
    ("no-fringes" . 0)
    ("right-only" . (0 . nil))
    ("left-only" . (nil . 0))
    ("half-width" . (4 . 4))
    ("minimal" . (1 . 1)))
  "Alist mapping fringe mode names to fringe widths.
Each list element has the form (NAME . WIDTH), where NAME is a
mnemonic fringe mode name and WIDTH is one of the following:
- nil, which means the default width (8 pixels).
- a cons cell (LEFT . RIGHT), where LEFT and RIGHT are
  respectively the left and right fringe widths in pixels, or
  nil (meaning the default width).
- a single integer, which specifies the pixel widths of both
fringes.")

Be careful when playing with the fringe size. Certain info doesn’t look very good when the fringe is too small (e.g. less than 4 pixels).