lower man page
lower — Change a window's position in the stacking order
lower window ?belowThis?
If the belowThis argument is omitted then the command lowers window so that it is below all of its siblings in the stacking order (it will be obscured by any siblings that overlap it and will not obscure any siblings). If belowThis is specified then it must be the path name of a window that is either a sibling of window or the descendant of a sibling of window. In this case the lower command will insert window into the stacking order just below belowThis (or the ancestor of belowThis that is a sibling of window); this could end up either raising or lowering window.
All toplevel windows may be restacked with respect to each other, whatever their relative path names, but the window manager is not obligated to strictly honor requests to restack.
lower, obscure, stacking order