window-combination-limit

window-combination-limit is a variable defined in `C source code'.
Its value is

window-size

Documentation:
If non-nil, splitting a window makes a new parent window.
The following values are recognized:

nil means splitting a window will create a new parent window only if the
window has no parent window or the window shall become part of a
combination orthogonal to the one it is part of.

`window-size' means that splitting a window for displaying a buffer
makes a new parent window provided `display-buffer' is supposed to
explicitly set the window's size due to the presence of a
`window-height' or `window-width' entry in the alist used by
`display-buffer'. Otherwise, this value is handled like nil.

`temp-buffer' means that splitting a window for displaying a temporary
buffer always makes a new parent window. Otherwise, this value is
handled like nil.

`display-buffer' means that splitting a window for displaying a buffer
always makes a new parent window. Since temporary buffers are
displayed by the function `display-buffer', this value is stronger
than `temp-buffer'. Splitting a window for other purpose makes a
new parent window only if needed.

t means that splitting a window always creates a new parent window. If
all splits behave this way, each frame's window tree is a binary
tree and every window but the frame's root window has exactly one
sibling.

Other values are reserved for future use.

You can customize this variable.

This variable was introduced, or its default value was changed, in version 24.3 of Emacs.