King Se-jong is known as the greatest king of Joseo Dynasty. He had done a lot of things when he was on the throne. He hired a slave named Jang Yeong-sil and he invented a lot of scientific devices such as water clock, sun clock etc. All of his inventions were devoted to the agricultural development in Joseon.  

  The main reason why King Se-jong is called the greatest king of Joseon Dyansty is bacause his invention of Korean alphabet. These letters are known to be created by the king himself, and it is written that he woked very hard to make the letters. He observed how every single vocal organ works when they produce sounds and made the shape of each letter similar to the shape of a vocal organ when it makes particular sound. Thus, Korean alphabetical system is believed to be the best way to express human sound.