Frown lines, also referred to as wrinkles, worry lines, forehead furrows, glabellar lines, or “the 11s,” are deep vertical lines that develop on the forehead. They may also develop on the corners of the mouth. Frown lines usually occur due to thinning skin, sun damage, and repeated movements of the muscles on your face. Frown lines are caused by a combination of factors, only some of which can be prevented by making changes to your lifestyle.