Origin of Hangul
Ledyard holds that the Koreans adopted five core consonant letters from ’Phags-pa, namely ㄱ "g" , ㄷ "d" , ㅂ "b" , ㅈ "j" , and ㄹ "l" . These were the consonants basic to Chinese phonology, rather than the graphically simplest letters (ㄱ "g" , ㄴ "n" , ㅁ "m" , and ㅅ "s" ) taken as the starting point by the "Haerye." A sixth letter, the null initial ㅇ, was invented by Sejong. The rest of the consonants were developed through featural derivation from these six, essentially as described in the "Haerye"; a resemblance to speech organs was an additional motivating factor in selecting the shapes of both the basic letters and their derivatives.
Hunminjeongeum
Hunminjeongeum (lit. "The Correct/Proper Sounds for the Instruction of the People") is a document describing an entirely new and native script for the Korean language. The script was initially named after the publication, but later came to be known as hangul. It was created so that the common people illiterate in hanja could accurately and easily read and write the Korean language. It was announced in Volume 102 of the "Annals of King Sejong", and its formal supposed publication date, October 9, 1446, is now Hangul Day in South Korea. The Annals place its invention to the 25th year of Sejong's reign, corresponding to 1443–1444.