When covering a window with curtains, most people think of two pieces of fabric, one on each side of a window. But most curtains are sold by the panel nowadays. A panel is a single curtain, so it is one piece of fabric. So if you want that traditional curtain look, you will need to buy two curtain panels each wide enough to cover at least half the width of the window or more. A rule of thumb when choosing the curtain panel width is to make sure each panel is wide enough to cover half the window with extra width to keep the look of fullness when the curtains are closed. If you prefer to have only one panel per window, just make sure the the panel is wide enough to cover the window completely.