27.6 Splitting Windows

This section describes functions for creating a new window by splitting an existing one.

Function: split-window &optional window size side pixelwise

This function creates a new live window next to the window window. If window is omitted or nil, it defaults to the selected window. That window is split, and reduced in size. The space is taken up by the new window, which is returned.

The optional second argument size determines the sizes of window and/or the new window. If it is omitted or nil, both windows are given equal sizes; if there is an odd line, it is allocated to the new window. If size is a positive number, window is given size lines (or columns, depending on the value of side). If size is a negative number, the new window is given −size lines (or columns).

If size is nil, this function obeys the variables window-min-height and window-min-width (see Window Sizes). Thus, it signals an error if splitting would result in making a window smaller than those variables specify. However, a non-nil value for size causes those variables to be ignored; in that case, the smallest allowable window is considered to be one that has space for a text area one line tall and/or two columns wide.

Hence, if size is specified, it’s the caller’s responsibility to check whether the emanating windows are large enough to encompass all areas like a mode line or a scroll bar. The function window-min-size (see Window Sizes) can be used to determine the minimum requirements of window in this regard. Since the new window usually inherits areas like the mode line or the scroll bar from window, that function is also a good guess for the minimum size of the new window. The caller should specify a smaller size only if it correspondingly removes an inherited area before the next redisplay.

The optional third argument side determines the position of the new window relative to window. If it is nil or below, the new window is placed below window. If it is above, the new window is placed above window. In both these cases, size specifies a total window height, in lines.

If side is t or right, the new window is placed on the right of window. If side is left, the new window is placed on the left of window. In both these cases, size specifies a total window width, in columns.

The optional fourth argument pixelwise, if non-nil, means to interpret size in units of pixels, instead of lines and columns.

If window is a live window, the new window inherits various properties from it, including margins and scroll bars. If window is an internal window, the new window inherits the properties of the window selected within window’s frame.

The behavior of this function may be altered by the window parameters of window, so long as the variable ignore-window-parameters is nil. If the value of the split-window window parameter is t, this function ignores all other window parameters. Otherwise, if the value of the split-window window parameter is a function, that function is called with the arguments window, size, and side, in lieu of the usual action of split-window. Otherwise, this function obeys the window-atom or window-side window parameter, if any. See Window Parameters.

As an example, here is a sequence of split-window calls that yields the window configuration discussed in Windows and Frames. This example demonstrates splitting a live window as well as splitting an internal window. We begin with a frame containing a single window (a live root window), which we denote by W4. Calling (split-window W4) yields this window configuration:

     ______________________________________
    | ____________________________________ |
    ||                                    ||
    ||                                    ||
    ||                                    ||
    ||_________________W4_________________||
    | ____________________________________ |
    ||                                    ||
    ||                                    ||
    ||                                    ||
    ||_________________W5_________________||
    |__________________W3__________________|

The split-window call has created a new live window, denoted by W5. It has also created a new internal window, denoted by W3, which becomes the root window and the parent of both W4 and W5.

Next, we call (split-window W3 nil 'left), passing the internal window W3 as the argument. The result:

     ______________________________________
    | ______  ____________________________ |
    ||      || __________________________ ||
    ||      |||                          |||
    ||      |||                          |||
    ||      |||                          |||
    ||      |||____________W4____________|||
    ||      || __________________________ ||
    ||      |||                          |||
    ||      |||                          |||
    ||      |||____________W5____________|||
    ||__W2__||_____________W3_____________ |
    |__________________W1__________________|

A new live window W2 is created, to the left of the internal window W3. A new internal window W1 is created, becoming the new root window.

For interactive use, Emacs provides two commands which always split the selected window. These call split-window internally.

Command: split-window-right &optional size

This function splits the selected window into two side-by-side windows, putting the selected window on the left. If size is positive, the left window gets size columns; if size is negative, the right window gets −size columns.

Command: split-window-below &optional size

This function splits the selected window into two windows, one above the other, leaving the upper window selected. If size is positive, the upper window gets size lines; if size is negative, the lower window gets −size lines.

User Option: split-window-keep-point

If the value of this variable is non-nil (the default), split-window-below behaves as described above.

If it is nil, split-window-below adjusts point in each of the two windows to minimize redisplay. (This is useful on slow terminals.) It selects whichever window contains the screen line that point was previously on. Note that this only affects split-window-below, not the lower-level split-window function.