Generally, an easy way to figure it out is to look at where the glass intersects the frame. On a single pane window you'll see the interior frame, a gap (the gl

Generally, an easy way to figure it out is to look at where the glass intersects the frame. On a single pane window you'll see the interior frame, a gap (the glass) and the exterior frame. On a double pane you'll see a gasket between the frames that separates the two panes. On a triple pane you'll see two gaskets separating the three panes. The gaskets maintain an airspace between the glass panes, which is responsible for most of the thermal performance. On new windows this can even be filled with a transparent gas (like argon or krypton) to increase performance.