Re: switch-to-buffer-other-window to split vertically by default ?

[martin rudalics]

With complete disregard for frames:

(defun my-display-buffer (buffer &optional not-this-window frame)
  (if (one-window-p)
      (let ((window (split-window-horizontally)))
        (set-window-buffer window buffer)
        window)
    (let (display-buffer-function)
      (display-buffer buffer not-this-window frame))))

(setq display-buffer-function 'my-display-buffer)