Set aside style considerations for a moment; function comes first and will limit your curtain choices, in a good way. If you want treatments that provide privacy or total darkness, you need lined curtains. If you’re OK with light filtering through or if your curtains are simply decorative, unlined will work. Lining is more expensive but has other advantages: It can shield fabric from sun damage, making curtains last longer. A lining also adds heft, which protects against drafts and helps fabric fall more luxuriously. For maximum durability, light blockage, body, and insulation, you can get curtains with an interlining, too—a layer of flannel-like fabric sewn between the lining and the “face” fabric. This is a common option for custom-made curtains but not widely available in less expensive ready-made panels.