Вот ещё хорошее объяснение https://annacurly770574943.wordpress.com/2018/01/14/%D0%BD%D0%B5%D0%BC%D0%BD%D0%BE%D0%B3%D0%BE-%D0%BE-%D1%81%D1%80%D0%BE%D0%BA%D0%B0%D1%85-%D0%B3%D0%BE%D0%B4%D0%BD%D0%BE%D1%81%D1%82%D0%B8/