MEASURE FROM INSIDE THE WINDOW TRACK! The Ideal Flexible Flap Side Sliding Window Insert is just like a patio insert for sliding glass doors; but made shorter for vertical windows. The top of the unit is spring loaded to give you a 2.5" adjustment range for easy fitting and removal. A locking pin is included that you may install on the frame of your sliding window. Most customers use a small stick behind the window in the track instead as they do not wish to install any hardware. All weather stripping is included as well as a bug protector (draftstopper) that is installed to cove the gap created when the window insert is installed.