Some items of clothing are designed to be worn as underwear, while others such as T-shirts can be used both as underwear and as outer clothing. Whether underwear can be used as outer clothing depends on a number of things, such as how warm it is indoors and outdoors, what other people in society find acceptable, fashion, and what the law allows. Some types of underwear can be used for sleeping in or as swimsuits, if they are made of suitable fabric.