An inner or under fabric for improving a curtain drape, supporting a weaker or more wayward fabric, and protecting from sunlight, insects and dust. Standard curtain linings are twill woven cotton–cotton sateen–in white and neutral tones, though they are also available in a wide range of colours from specialist suppliers. They will always be treated at source to prevent damage by moth and sunlight, and some against fire and water.
However, any fabric can be used for lining: it simply needs to be appropriate to factors such as such as budget, climate, room, etc.
I prefer to see an interesting lining, so our standard is English or French woven cotton ticking with soft stripes, which adds a little something to the finished whole.