Chinese opera is a popular form of drama and musical theatre in China with a history of over 1000 years. It evolved from early simple forms of Chinese drama that incorporated music, song, dance, martial arts and acrobatics. By the 13th century during the Song Dynasty, Chinese opera had reached its mature form. Different regional operas also developed, such as Beijing Opera and Sichuan Opera. Ancient Chinese used a sundial-like instrument called a "rigui" to divide the day into 12 hours represented by 12 animals.