The revolutionary new window type that emerged from the London joiners’ workshops during the period c. 1670 to c. 1685 (see Part One) owes its existence to two novel features in particular, the introduction of the glazed wooden sliding frame or ‘sash’, and the application of a counterbalancing mechanism. The first technique, a French innovation of about half a century earlier, greatly enhanced the weather-proofing capacity of the window and, because of its rigidity, also permitted a more effective exploitation of advances made in the manufacturing of large sheets of clear flatglass (for mirrors & windows) than had previously been the case. The second, an English invention, released the full potential of the new wooden frame and encouraged the development of a large openable window which, in terms of convenience and technical efficiency, was superior to anything else on the market. In order to understand why this development took the course it did we need to look briefly at the general state of fenestration in English architecture at the time of the sash-window’s appearance.