— User Option: window-sides-vertical

If non-nil, the side windows on the left and right of a frame occupy the frame's full height. Otherwise, the side windows on the top and bottom of the frame occupy the frame's full width.

— User Option: window-sides-slots

This option specifies the maximum number of side windows on each side of a frame. The value is a list of four elements specifying the number of side window slots on (in this order) the left, top, right and bottom of each frame. If an element is a number, it means to display at most that many windows on the corresponding side. If an element is nil, it means there's no bound on the number of slots on that side.

If any of the specified values is zero, no window can be created on the corresponding side. display-buffer-in-side-window will not signal an error in that case, but will return nil. If a specified value just forbids the creation of an additional side window, the most suitable window on that side is reused and may have its window-slot parameter changed accordingly.

— User Option: window-sides-reversed

This option specifies whether top/bottom side windows should appear in reverse order. When this is nil, side windows on the top and bottom of a frame are always drawn from left to right with increasing slot values. When this is t, the drawing order is reversed and side windows on the top and bottom of a frame are drawn from right to left with increasing slot values.

When this is bidi, the drawing order is reversed if and only if the value of bidi-paragraph-direction (see Bidirectional Display) is right-to-left in the buffer displayed in the window most recently selected within the main window area of this frame. Sometimes that window may be hard to find, so heuristics are used to avoid that the drawing order changes inadvertently when another window gets selected.

The layout of side windows on the left or right of a frame is not affected by the value of this variable.

— Function: window-main-window &optional frame

This function returns the main window of the specified frame. The optional argument frame must be a live frame and defaults to the selected one.

If frame has no side windows, it returns frame's root window. Otherwise, it returns either an internal non-side window such that all other non-side windows on frame descend from it, or the single live non-side window of frame. Note that the main window of a frame cannot be deleted via delete-window.

— Command: window-toggle-side-windows &optional frame

This command toggles side windows on the specified frame. The optional argument frame must be a live frame and defaults to the selected one.

If frame has at least one side window, this command saves the state of frame's root window in the frame's window-state frame parameter and deletes all side windows on frame afterwards.

If frame has no side windows, but does have a window-state parameter, this command uses that parameter's value to restore the side windows on frame leaving frame's main window alone.

An error is signaled if frame has no side windows and no saved state is found for it.