Tap water (running water, city water, municipal water, etc.) is water supplied to a tap (valve). Its uses include drinking, washing, cooking, and the flushing of toilets. Indoor tap water is distributed through “indoor plumbing”, which has existed since antiquity but was available to very few people until the second half of the 19th century, when it began to propagate in what are now the developed countries. It became common in many regions during the 20th century, and is now lacking only among the poor, especially in developing countries. Tap water is often culturally assumed to be potable water (drinking water), especially in developed countries. Usually it is potable, although water quality problems are not rare. Household water purification methods such as water filters, boiling, or distillation can be used when the potability is doubted. The application of technologies (such as water treatment plants) involved in providing clean water to homes, businesses, and public buildings is a major subfield of sanitary engineering. Calling a water supply “tap water” distinguishes it from the other main types of fresh water which may be available; these include water from rainwater-collecting cisterns, water from village pumps or town pumps, or water carried from streams, rivers, or lakes (whose potability may vary).