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.
Choe Sejin
Choe Sejin is mostly known for his 1527 work, "Hunmong Jahoe" (훈몽자회). It is a textbook for children to learn the Chinese characters, and is known to be very practical in its teachings. During the Joseon Era, literacy in Chinese was an essential skill for advancement in society. Chinese was the dominant language of literature at this time, and children were taught Chinese from an early age to prepare for the future. Although hangul, the Korean orthography, existed by this time, it was not widely used in the country. Choe wanted to promote the usage of hangul through his work "Hunmong Jahoe". As a textbook for children to learn Chinese, "Hunmong Jahoe" incorporated hangul in the textbook to promote both the learning of Chinese characters, as well as hangul. The Chinese characters were annotated in hangul, and in order for one to understand and learn the Chinese characters, one must have a full grasp of hangul first to comprehend the annotations. Choe wanted people to use hangul more extensively, and he thought that this would encourage people to take some time to fully learn hangul, before starting to learn Chinese. He wrote that "it would take [only a single day] to learn Hangul", commenting on the simplicity of hangul, and afterwards, one would be able to learn Chinese on their own without an instructor if one knew hangul and utilized his textbook.