Function: windmove-down

Select the window below the current one.
With no prefix argument, or with prefix argument equal to zero,
"down" is relative to the position of point in the window; otherwise
it is relative to the left edge (for positive ARG) or the right edge
(for negative ARG) of the current window.
If no window is at the desired location, an error is signaled. (fn &optional ARG)