If you think of a typical bedroom window, you will have two panes of glass where one slides over the other along the track to let fresh air in. In a case such as this, you would have two panes of glass or one window. Think of the window as the cut out from the structure of your house and the panes as the pieces of glass. We can typically provide you a quote with either number, we just need to know which way you are counting. If you count by pane though your estimate will likely be more accurate.