Useful information on the Sash Window History by the English Heritage.

Single-hung sash windows (where only the bottom frame moves and is supported by wooden gates or pins when open) were probably copied by the British from France some time in the mid-seventeenth century. The double-hung sash, however, appears to have been an British invention. This was an ingenious technological breakthrough that enabled a far more subtle and sophisticated system of ventilation to be achieved than was possible with the old, side-hung casement. It used a system of hidden, counterbalanced weights to allow both top and bottom sash frames to be moved independently. The earliest double-hung sash discovered so far dates from 1701; however, by 1720 double-hung sashes had spread only as far as Holland and the British and Dutch colonies.Until the early eighteenth century sash frames were usually made of native oak or a similar hardwood. By 1700, oak was becoming rarer and thus more expensive, while softwoods from Scotland, the Baltic, and North America were more widely available. From the 1720′s most sash joinery used deal, a generic term for pine or fir softwood.

As a precautionary measure against fire the 1709 Building Act required that the corners of a sash box frame be hidden behind the face of the brick or stone masonry and that ‘no door or window frame of wood… shall be set nearer to the outside face of the wall than four inches’. In 1774 this distance was increased to nine inches, and nearly all of the frame had to be hidden behind the face of the wall. While this legislation was initially only applicable to London, within a few years its provisions were being taken up throughout Britain and America.

By 1730 segmentally arched windows had largely been replaced by square-headed varieties that were cheaper to make. The glazing patterns inserted into these frames often took the form of six panes over six, although this was by no means the rule. Nor were the dimensions of each pane necessarily dependent on the principle of the golden section: in some cases, individual panes were broader than they were tall. The overall size of the window was, nevertheless, always kept in strict proportional harmony with the rest of the facade, in accordance with Palladian theory.

Early glazing bars were thick and robust, to support and protect the fragile glass. Most seventeenth – and early eighteenth-century examples were based on the ovolo, or quarter-circle moulding. As glass technology improved and glass became stronger, glazing bars became increasingly slender, with pointed (gothic) and lamb’s tongue mouldings becoming very popular. By 1820 some glazing bars were only 12.5mm wide (although, to provide lateral strength, these could be up to 38mm deep).

As with so many elements of the Georgian house, glazing-bar patterns and profiles varied according to the social status of the window. Thus, for example, basement or attic windows, used only by servants, were often fitted with old-fashioned, obtrusive ovolo glazing bars and inferior-quality glass.

With the introduction of cheap, strong plate glass in the 1830′s glazing bars became less necessary and views became accordingly less cluttered. By 1850 many sash-window frames had no internal glazing at all. However, the great weight of these frames and the absence of any internal supports necessitated the introduction on the upper frame of sash horns, extensions of the stiles which helped to strengthen the vulnerable joints at either end of the meeting rail.