Sejong the Great (May 6, 1397 – May 18 1450) was the fourth king of the Joseon Dynasty. He is one of only two Korean kings called "great" (the other is Gwanggaeto the Great of Goguryeo). He picked out young scholars and tried to fix the prejudice of classes by helping intelligent but low-class prodigies. In 1442, he had Jang Yeongsil, a scientist in Joseon, invent the Cheugugi (Hangul: 측우기; Hanja: 測雨器). This was a kind of tool to help measure the rain and how much it fell. Also, he expanded the territory of Joseon.
One of the best works and the biggest reason why Sejong was called great was because he created the Hunminjeongeum (Hangul: 훈민정음; Hanja: 訓民正音). It is called in modern days as Hangul (Korean: 한글). Because of the creation of the Hunminjeongeum, Joseon (now Korea) now got its own alphabet, and many people, most of whom have never learned how to read Hanja (Chinese characters used for Korean) could learn how to read and write much more easily than before.