Casement windows took over from the Sliding Sash as they were easier and cheaper to produce, required less maintenance, and had more of a modern look about them. Different to the Sliding Sash with its hollow box construction, the casement window is made up of a solid timber rebated frame. The joints are mortice and tenon, with openers hinged to the rebates.
The casement window has many different configurations; where the openers can be top, side hung, or fixed. Originally, the style was to have top openers with side hung casements at the bottom, where the larger wider frames would have mullions to divide up the expanse with side hung and fixed casements.
Eventually the design changed to where the bottom openers were removed, and replaced with a single sheet of glass, producing the now well known 'Picture Window' style. However, because the whole casement fitted into the rebate, this design had its problems, especially when not regularly maintained. This was due mainly to the fact that casements opened outward, leaving the casement, and rebate exposed to the elements when closed. This caused the opener to swell and twist.
To overcome this problem the design was modified into the stormproof version. The stormproof variant saw the rebate of the frame reduced, so that the casement opener protruded outside the frame. This protrusion allowed the larger casement to be rebated to its outer edge so that it fitted over and up to the frame, as shown in the detail below. This also meant the need for a different type of hinge to be used, the cranked stormproof hinge, a hinge that would pivot the casement away from the frame due to the rebate that would otherwise bind to the frame.
Shown below are the the 'cut through' sections of the of the stormproof window, with the additional pictures of the open casement.