For covering windows however single panel drapes have a variety of uses.
One window panel or two.
It has two panels where one or both of the panels are slided left and right to allow ventilation into the room.
The first thing to consider in dressing a single window is size.
Of course you can always buy 2 to start and see how you like the look of only 1 panel each window first.
Because there is no additional panes of glass single pane windows do not offer as much as double or triple pane windows by way of insulation to the home and may not be as good at keeping out noise.
The 2 panel slider is the most common type of sliding window.
Most of the time curtains come in pairs.
If you want the curtains to be able to close fully you should always use enough panels to cover the width of the window.
For example if you have a 28 inch wide panel and a 36 inch curtain rod purchase two panels for minimum fullness three panels for medium fullness and four panels for extra fullness.
This single sheer panel fits the window perfectly and can be drawn back to let in light and air.
Suitable for narrow outside spaces.
Several single panels can be combined to curtain an extra wide window.
Round up to the next whole number if you want to achieve more fullness.
Give the window a look that makes it feel proportional to the room.
Double pane windows have two panes of glass many with insulating argon gas between the panes.
If you did only one what would you put in the 3 space between them.
My personal preference with that much space between the windows would be for two panels per window.