Setting Emacs to Split Buffers Side-by-Side

(setq split-height-threshold nil)
(setq split-width-threshold 0)

GNU Emacs Lisp Reference Manual: Choosing Window Options

Two solutions here, use any one you like:

A: Vertically(left/right) by default:

(setq split-height-threshold nil)
(setq split-width-threshold 0)

B: Automatically split window vertically(left/right) if current window is wide enough

(defun display-new-buffer (buffer force-other-window)
  "If BUFFER is visible, select it.
If it's not visible and there's only one window, split the
current window and select BUFFER in the new window. If the
current window (before the split) is more than 100 columns wide,
split horizontally(left/right), else split vertically(up/down).
If the current buffer contains more than one window, select
BUFFER in the least recently used window.
This function returns the window which holds BUFFER.
FORCE-OTHER-WINDOW is ignored."
  (or (get-buffer-window buffer)
    (if (one-window-p)
        (let ((new-win
               (if (> (window-width) 100)
                   (split-window-horizontally)
                 (split-window-vertically))))
          (set-window-buffer new-win buffer)
          new-win)
      (let ((new-win (get-lru-window)))
        (set-window-buffer new-win buffer)
        new-win))))
;; use display-buffer-alist instead of display-buffer-function if the following line won't work
(setq display-buffer-function 'display-new-buffer)

Put any one in you .emacs/init.el file. You can change the "100" to the value you like, depending on you screen.

If you got two windows in one frame, and you want to change the layout from vertical to horizontal or vice verse, here is a solution:

(defun toggle-window-split ()
  (interactive)
    (if (= (count-windows) 2)
      (let* ((this-win-buffer (window-buffer))
            (next-win-buffer (window-buffer (next-window)))
            (this-win-edges (window-edges (selected-window)))
            (next-win-edges (window-edges (next-window)))
            (this-win-2nd
             (not (and (<= (car this-win-edges)
                        (car next-win-edges))
                    (<= (cadr this-win-edges)
                        (cadr next-win-edges)))))
         (splitter
          (if (= (car this-win-edges)
                 (car (window-edges (next-window))))
              'split-window-horizontally
            'split-window-vertically)))
    (delete-other-windows)
    (let ((first-win (selected-window)))
      (funcall splitter)
      (if this-win-2nd (other-window 1))
      (set-window-buffer (selected-window) this-win-buffer)
      (set-window-buffer (next-window) next-win-buffer)
      (select-window first-win)
      (if this-win-2nd (other-window 1))))))
;; C-x 4 t 'toggle-window-split
(define-key ctl-x-4-map "t" 'toggle-window-split)

Put it in your .emacs/init.el file, Use C-x 4 t to toggle the layout of your windows.

(setq split-height-threshold 0) (setq split-width-threshold 0)

is what i had to use to get the desired behaviour (no horizontal splitting)

Sometimes we need change between Horizontal and Vertical according current display and our requirement (more lines or more columns).

I recommand the great ToggleWindowSplit, And I bind key to "C-c y"

http://www.emacswiki.org/emacs/ToggleWindowSplit

the simple answer of setting 2 variables to nil and 0 didn't work for me, so I wrote 2 simple functions: one just splits the window into NX vertical buffers and opens files named (for example) file.1 file.2 ... file.NX in each and another one does the same think, except does it in 2D (NY rows by NX columns for opening files f.1 f.2 ... f.[NX*NY]). To install, add this code to .emacs:

    (defun grid-files-h (nx wx pfx)
  "Using dotimes, split the window into NX side-by-side buffers of width WX and load files starting with prefix PFX and ending in numbers 1 through NX"
  (let (ox fn k)  ; ox is not used, but fn is used to store the filename, and k to store the index string
    (dotimes (x (- nx 1) ox) ; go through buffers, x goes from 0 to nx-2 and ox is not used here
;     (print x)
      (setq k (number-to-string (+ x 1) ) )  ; k is a string that goes from "1" to "nx-1"
;     (print k)
      (setq fn (concat pfx k) ) ; fn is filename - concatenate prefix with k
;     (print fn)
      (find-file fn) ; open the filename in current buffer
      (split-window-horizontally wx) ; split window (current buffer gets wx-columns)
      (other-window 1) ; switch to the next (right) buffer
      )
    (setq k (number-to-string nx )) ; last (rightmost) buffer gets the "nx" file
    (setq fn (concat pfx k) ) ; fn = "pfx"+"nx"
    (find-file fn ) ; open fn
    (other-window 1) ; go back to the first buffer
    )  
  )

   (defun grid-files-sq (ny wy nx wx pfx)
      "Using dotimes, split the window into NX columns of width WX and NY rows of height WY and load files starting with prefix PFX and ending in numbers 1 through NX*NY"
      (let (oy ox fn k)  
        (dotimes (y ny oy) ; go through rows, y goes from 0 to ny-1 and oy is not used here
          (split-window-vertically wy) ; create this row
          (dotimes (x (- nx 1) ox) ; go through columns, x goes from 0 to nx-2 and ox is not used here
        (setq k (number-to-string (+ 1 (+ x (* y nx) ) ) ) ) ; k must convert 2 indecies (x,y) into one linear one (like sub2ind in matlab)
        (setq fn (concat pfx k) ) ; filename
        (find-file fn ) ; open
        (split-window-horizontally wx) ; create this column in this row (this "cell")
        (other-window 1) ; go to the next buffer on the right 
        )
          (setq k (number-to-string (+ nx (* y nx) ) ) ) ; rightmost buffer in this row needs a file too
          (setq fn (concat pfx k) ) ; filename
          (find-file fn ) ; open
          (other-window 1) ; go to next row (one buffer down)
          )
        )
      )

https://i.stack.imgur.com/2YPmw.png

This probably can be done better/more efficiently, but it's a start and it does what I need it to do (display a bunch of sequentially named small files). Feel free to improve or reuse.

I use multiple frames (OSX windows) in emacs regularly for different projects. Here's how I setup a few frames initially split to a left and right window.

  (defun make-maximized-split-frame (name)
    (let (( f (make-frame (list (cons 'name  name))) ))
      (maximize-frame f)
      (split-window (frame-root-window f) nil t)
      ))

  (make-maximized-split-frame "DocRaptor")
  (make-maximized-split-frame "Gauges")
  (make-maximized-split-frame "Instrumental")

The direct answer is to press C-c 3.

It's not clear from the question if you want a permanent setting change, but I found this question looking for this answer and didn't find it. (The answer has actually been sitting in a comment for the last 11 years)