a convertible window system having two window sashes , a weatherstripping , and a tilt pivot assembly . the window system can be readily switched between single - hung and double - hung configuration . the weatherstripping assembly , which is operatively connected to each of the two sashes , has a number of regions . each of these regions has different attributes which determine the types of translations and rotations that can be performed on a sash with a predetermined force . in a preferred embodiment , an upper sash can be removed from a window frame by the application of a predetermined force on the top of the sash while the sash is within a predetermined range of the weatherstripping assembly without the need for withdrawing guide pins from the channel of the weatherstripping assembly .