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.
Sin Sukju
Shin was an accomplished polyglot, and was particularly well educated in the Chinese language. He served as a personal linguistic expert to King Sejong, and was intimately involved in the creation and application of the Korean alphabet known in modern times as Hangul. Sin used the newly created hangul system to create an accurate transcription of spoken Mandarin Chinese in 15th century Ming dynasty China.
These transcriptions haven proven accurate and reliable, and his transcriptions are now "an invaluable source of information on the pronunciations of Ming-era [Mandarin]."