DATING OF WINDOWS
Windows ( or wind-eyes ) were originally to let in air and let out smoke and were unglazed well into the 16th century. Early windows were closed with timber shutters, the windows were then closed with metal frame casements with paper, linencloth or parchment; sheepskin or sacking would be more appropriate in country cottages. Glass came into being in the form of diamond shaped "quarries" cut from blown bubbles, set into metal frames and lead cames.
The early 17th century saw the arrival of larger panes of glass or crown glass, cut from a spun disc. The sash window was introduced in the second half of the 17th century ( from Holland ) and was confined to the grandest of houses until the early 18th century, due to the expense. The oldest original sash windows date from c 1730. More humble accommodation resorted to timber copies of the metal frames, being casements, and would have remained so until the late 18th century. However, it was more usual for these windows to be four pane casements.
Another clue as to the dating of existing windows is concerned, rests with the horns on sashes, these not being introduced until the late 1840s, due to the strength required to make the joints for the heavier glass, the peg dovetail being inadequate. Extended horns on these windows would indicate a more specific purpose ( of a limited opening ) and therefore would be very much later, 1870-1900.
It is amazing just how many times the TV and Film industry gets this wrong.