context: At the end of the 12th century and the beginning of the 13th century In France, the so-called Notre Dame school was born. Its principal representatives were the composers Perotinus (or Pérotin) and Leoninus (or Léon), both of whom had studied music at the University of Paris and worked at Notre Dame Cathedral. An anonymous English manuscript from 1275 tells Léon that he wrote Magnus Liber's organ, which contained two-tone (duplum Organum) compositions for all the responsive parts of the church's great celebration of worship. Although the book itself has been lost, many manuscripts have preserved parts of it. Later, Pérot and other school composers edited and expanded the collection and made alternative parts and triplum organum or quatrum organum arrangements for different works. [12]
question: When was the Notre Dame School founded?
answers.text: At the end of the 12th century and the beginning of the 13th century