Window Grille Inside the window frame may be smaller sections of window that go beneath distinct terms but collectively called grillework. More mature, solitary-pane windows may have legitimate mullions, where panes are separated by Wooden. These strips are often well suited for draught-proofing a window with casements but not sash designs.  https://reidytjuf.blog-a-story.com/16836539/sash-window-draught-proofing-an-overview