What is the difference between frame size and aperture size?

The aperture (or structural opening) is the hole in the brickwork that the window sits into. The frame size is the actual outer dimension of the manufactured window, which is always smaller than the aperture to leave a gap for fitting, packing and sealing. When ordering supply-only, be clear which one you’re quoting: order to the finished frame size you want manufactured, having already deducted your fitting clearance normally of approximately 10mm from the aperture. Confusing the two is the most common cause of a window that won’t fit.