split-window

split-window is a compiled Lisp function in `window.el'.

(split-window &optional WINDOW SIZE SIDE PIXELWISE)

Make a new window adjacent to WINDOW.
WINDOW must be a valid window and defaults to the selected one.
Return the new window which is always a live window.

Optional argument SIZE a positive number means make WINDOW SIZE
lines or columns tall. If SIZE is negative, make the new window
-SIZE lines or columns tall. If and only if SIZE is non-nil, its
absolute value can be less than `window-min-height' or
`window-min-width'; so this command can make a new window as
small as one line or two columns. SIZE defaults to half of
WINDOW's size.

Optional third argument SIDE nil (or `below') specifies that the
new window shall be located below WINDOW. SIDE `above' means the
new window shall be located above WINDOW. In both cases SIZE
specifies the new number of lines for WINDOW (or the new window
if SIZE is negative) including space reserved for the mode and/or
header line.

SIDE t (or `right') specifies that the new window shall be
located on the right side of WINDOW. SIDE `left' means the new
window shall be located on the left of WINDOW. In both cases
SIZE specifies the new number of columns for WINDOW (or the new
window provided SIZE is negative) including space reserved for
fringes and the scrollbar or a divider column. Any other non-nil
value for SIDE is currently handled like t (or `right').

PIXELWISE, if non-nil, means to interpret SIZE pixelwise.

If the variable `ignore-window-parameters' is non-nil or the
`split-window' parameter of WINDOW equals t, do not process any
parameters of WINDOW. Otherwise, if the `split-window' parameter
of WINDOW specifies a function, call that function with all three
arguments and return the value returned by that function.

Otherwise, if WINDOW is part of an atomic window, "split" the
root of that atomic window. The new window does not become a
member of that atomic window.

If WINDOW is live, properties of the new window like margins and
scrollbars are inherited from WINDOW. If WINDOW is an internal
window, these properties as well as the buffer displayed in the
new window are inherited from the window selected on WINDOW's
frame. The selected window is not changed by this function.