Early barns rarely had windows, but, by the early 19th century, Vermont farmers began installing windows on their barns to obtain more light in the interior.
Early 19th century windows usually consisted of transom lights above main, south-facing doors. A transom light is a long, rectangular window the sits right atop the door frame, seen in the image below.
Shortly thereafter, farmers began adding windows to the cow and horse stables. These windows were four or six-paned fixed windows that were usually fixed in place (unable open) but occasionally hinged. The image on the left illustrates a fixed six-paned window. The image on the right demonstrates the small size of these early windows.
In the mid-19th century, six-over-six, double-hung sash windows became popular. Double-hung sash windows have two window frames that slide up and down. Six-over-six windows have six panes in each sash, seen in the image below. By the 1890s, many two-over-two, double-hung sash windows were installed, but the six-over-six windows remained popular, as they were more durable and easier to repair.
Monitor roofs were built on barns in the late 19th and early 20th centuries for additional light and ventilation. The monitor is a raised portion of the roof with rows of narrow windows on each side, seen in the image below.
By the 1920s, rows of machine made steel or wood windows were installed in stables. They were either fixed or pivoted open. Notice the uniformity of the factory-made windows in the image below.