Steel or iron. In this image, the window panes are slightly vertical in orientation, but the overall look appears square-ish. Thicker mullions create additional partitions for a one-over-three window effect. In this design, the windows do all the talking; the view beyond is framed only by the panes of glass — there is no additional casing around the windows, which lightens the dark-paned windows and the rest of the beautiful room.
I quite like how the grid of squares is broken by the mansard window opening in this design; it's a stunning detail for an already gorgeous treatment of windows.
Another casing-less treatment, this lovely window/door-scape takes a traditional use of materials and translates them into a more modern look. The horizontal rectangles of glass are larger and more open than in the previous photo, which opens up the room further to the outside while still retaining a classic feel.
In this case, the window could have been a picture window, but the builder or homeowner chose against it.
In this special casement window, diagonal muntins separate diamond-shaped panes of glass. A deep and artful treatment of the wall around this true-arch window gives the whole vignette more depth.
In this case, each panel of windows is separated by a mullion, a vertical structural element that divides adjacent window units.
The pattern introduced by a transom window above this door is subtle and distinct. It adds tremendously to the transition between the rooms, not only in function (bringing light from one room to the next) but in beauty.
Natural wood. In wooden windows, a fillet is cut into the outer edge of the muntin to "stop" the pane of glass in the opening, and putty or thin strips of wood or metal are then used to hold the glass in place. The thickness of window muntins ranges from very slim (a product of 19th century Greek-revival buildings) to thick (influenced by 17th and early 18th century buildings).
This casement window, with elongated pieces of glass divided by natural wood muntins, infuses a serene, modern look in this bright bathroom.
Painted wood. The wood built-ins offset the bright white of the window wall and its multi-framed panels.
The muntins in this window design are a little thicker, while the glass panes are larger, allowing for more light to penetrate and less obstruction of the view.
Drywall mullions bring an architectural element to an otherwise pared-down space. The thick, framed windows make for a fantastic feature wall in this dining room.