(setq split-height-threshold nil) (setq split-width-threshold 200)

• Setting split-height-threshold to basically never wanting to split horizontally
• 200 seems like a high-enough number that even on a large external display Emacs will only split at most once.

This works for me on both my 15" MacBook Pro and external widescreen display:

(setq split-height-threshold nil) (setq split-width-threshold 200)

• Setting split-height-threshold to basically never wanting to split horizontally
• 200 seems like a high-enough number that even on a large external display Emacs will only split at most once.