Split the selected window into two windows, one above the other. Split the selected window into two windows positioned side by side. Both windows start out displaying the same buffer, with the same value of point. Shift and the middle mouse button causes the window to be split at the position of the mouse pointer. A numeric argument specifies how many columns to give the one on the left.
Scroll bars separate the two windows.
If you use the mouse, the window will be split at the position of the mouse pointer.