There are a variety of different types of mounting hardware used for screens.
If you are replacing an existing insect screen, you may notice with many of them that the top of the screen 'clips' to the middle bar of the window. They do this with what is known as a 'Ram's Horn Clip'. If your current screens have this type of mounting hardware, please make note of it when you order.
For screens that mount inside the window but do not fit into a 'track', as described above, the typical mounting hardware is either a Spring Plunger, Acorn Turn Latches, Swivel Latches, Slide Latches, Butterfly Latches or Flush Casement Clips (see Casement Clips below):
For screens that are 'flush-mount', basically mount to the outside of the window, there are a few different options available for Mounting Hardware.
Casement Clips - These screw directly into the window frame and hold the screen on. They come in 3 different sizes: flush, 5/16' and 7/16'. The 5/16' and 7/16' correspond to the frame size that you will order; and are used in situations where the screen will sit on the outside of the window frame. The flush casement clip is used in situations where the screen fits inside the window frame, but not into a 'track'.
Wire Loops w/ Ball Head Screws - These are typically used with wood windows. The loops are mounted on the bottom of the screen and slide over a Ball Head screw that is attached to the bottom of the window casement. These are used in conjunction with Jiffy Hangers/Friction Clips.
Window Screen Resource Center
The resources below provide information and articles meant to educate and answer any questions you may have related to window screens.