During the Age of the Three Kingdoms (57 B.C.-A.D. 676) there were significant musical advances on the Korean peninsula, including the Goguryeo nation's invention of the 'geomungo' and the Gaya nation's invention of the 'gayageum'. There was also an influx of music culture from China and its bordering countries as a part of the cultural exchange taking place at the time. During the Goryeo era, 'aak' and 'hyangak' gained influence- 'aak' being ceremonial court music played during the Jongmyo Shrine's Jerye Ceremony with Chinese roots, and 'hyangak' being the music of Goguryeo, Baekje, and Silla passed down directly to Goryeo. In the early/mid Joseon era (1392-1910) court music was in the spotlight, while in later years the focus was on folk music. 'Aak', being court music, was usually played during court ceremonies, and contained lyrics in praise of the king. In the 15th century, the music text 'Akhakgwebeom' was published, detailed music theory, history, and instruments. In the late Joseon era with the rapid evolution of commerce and industry and the increase in agricultural production levels, common culture began to develop, and music was directly influenced by such changes. In other words, there was an increase in members of society that began to enjoy music, broadening the types of musical genres available. Common culture saw the rise of the 'pansori' opera and folk music. Folk music was an honest, frank portrayal of the commoners' feelings and thoughts at the time, and many folk songs were a satire against the inequalities of society. This type of folk music later developed into 'samulnori' (music played on 4 different percussion instruments) and gained wide popularity with the general populace.