почитайтеhttp://ru.wikipedia.org/wiki/%D0%9A%D0%B8%D1%82%D0%BE%D0%B1%D0%BE%D0%B9%D0%BD%D1%8B%D0%B9_%D0%BF%D1%80%D0%BE%D0%BC%D1%8B%D1%81%D0%B5%D0%BB_%D0%BD%D0%B0_%D0%A4%D0%B0%D1%80%D0%B5%D1%80%D1%81%D0%BA%D0%B8%D1%85_%D0%BE%D1%81%D1%82%D1%80%D0%BE%D0%B2%D0%B0%D1%85