http://ru.wikipedia.org/wiki/%D0%A1%D0%BF%D0%B5%D1%86%D0%B8%D0%B0%D0%BB%D1%8C%D0%BD%D1%8B%D0%B5_%28%D0%BA%D0%BE%D1%80%D1%80%D0%B5%D0%BA%D1%86%D0%B8%D0%BE%D0%BD%D0%BD%D1%8B%D0%B5%29_%D0%BE%D0%B1%D1%80%D0%B0%D0%B7%D0%BE%D0%B2%D0%B0%D1%82%D0%B5%D0%BB%D1%8C%D0%BD%D1%8B%D0%B5_%D1%83%D1%87%D1%80%D0%B5%D0%B6%D0%B4%D0%B5%D0%BD%D0%B8%D1%8FИзучите 7 вид.