Vertical split shows more of each line, horizontal split shows more lines. This code toggles between them. It only works for frames with exactly two windows. The top window goes to the left or vice-versa. I was motivated by ediff-toggle-split and helped by TransposeWindows. There may well be better ways to write this.

(defun toggle-window-split ()
  (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)))))
	      (if (= (car this-win-edges)
		     (car (window-edges (next-window))))
	(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))))))

(define-key ctl-x-4-map "t" 'toggle-window-split)


Here’s a simpler implementation I wrote before knowing about this wiki page:

[This version doesn’t always preserve the associated buffers as the above version does]

(defun toggle-frame-split ()
  "If the frame is split vertically, split it horizontally or vice versa.
Assumes that the frame is only split into two."
  (unless (= (length (window-list)) 2) (error "Can only toggle a frame split in two"))
  (let ((split-vertically-p (window-combined-p)))
    (delete-window) ; closes current window
    (if split-vertically-p
      (split-window-vertically)) ; gives us a split with the other window twice
    (switch-to-buffer nil))) ; restore the original window in this part of the frame

;; I don't use the default binding of 'C-x 5', so use toggle-frame-split instead
(global-set-key (kbd "C-x 5") 'toggle-frame-split)


I wrote a version that works when there’re more than 2 windows (besides the minibuffer window):

(defun window-toggle-split-direction ()
  "Switch window split from horizontally to vertically, or vice versa.

i.e. change right window to bottom, or change bottom window to right."
  (require 'windmove)
  (let ((done))
    (dolist (dirs '((right . down) (down . right)))
      (unless done
        (let* ((win (selected-window))
               (nextdir (car dirs))
               (neighbour-dir (cdr dirs))
               (next-win (windmove-find-other-window nextdir win))
               (neighbour1 (windmove-find-other-window neighbour-dir win))
               (neighbour2 (if next-win (with-selected-window next-win
                                          (windmove-find-other-window neighbour-dir next-win)))))
          ;;(message "win: %s\nnext-win: %s\nneighbour1: %s\nneighbour2:%s" win next-win neighbour1 neighbour2)
          (setq done (and (eq neighbour1 neighbour2)
                          (not (eq (minibuffer-window) next-win))))
          (if done
              (let* ((other-buf (window-buffer next-win)))
                (delete-window next-win)
                (if (eq nextdir 'right)
                (set-window-buffer (windmove-find-other-window neighbour-dir) other-buf))))))))


(Note: This version does not seem to work with Agenda windows, but JeffDwork’s version does --mediapathic)