mouse-split-window-horizontally

mouse-split-window-horizontally is an interactive compiled Lisp function in `mouse.el'.

It is bound to <mode-line> <C-mouse-2>.

(mouse-split-window-horizontally CLICK)

Select Emacs window mouse is on, then split it horizontally in half.
The window is split at the column clicked on.
This command must be bound to a mouse click.