Sash
A sash onto a window consists of a pane or many panes of glass and the total framework comprising the panes. The framework of a sash consists of muntins, rails and stiles.
A sash can travel vertically in channels on the left and right sides of the window frame. A single-hung window is composed of a fixed top sash and movable bottom sash. On a double-hung window, the top and bottom sash can move.
Chris Hill
The lower sash is positioned closest to the interior of a room.
Britannia Joinery
Windows
The top sash is positioned closest to the outside.
Bosworth Hoedemaker
Open the sash and air is allow in by you.