You've go the glazing, the frame, the sash, the different parts of the glass that work together to make it possibly so you can open the window and receive some of that... it needs to be resistant enough to light, to be energy efficient but it needs to allow enough light so that you can see out of the window or else why would you have a window in the first place and it needs to be able to be sealed enough to act like a wall so that it stops the air from coming through but it needs to open enough it acts like a window so that you can have the air come through when you want it to. 