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.
Gari Ledyard
The "Hunmin Jeong-eum" credits the 古篆字 ""Gu" Seal Script" as being the source King Sejong or his ministers used to create hangul. This has traditionally been interpreted as the "Old" Seal Script, and has confused philologists because hangul bears no functional similarity to the Chinese seal scripts. However, 古 "gǔ" had more than one meaning: besides meaning "old", it could be used to refer to the Mongols (蒙古 "Měng-gǔ"). Records from Sejong's day played with this ambiguity, joking that "no one is more "gu" than the "Meng-gu"". That is, "Gu Seal Script" may have been a veiled reference to the Mongol Seal Script, or Phagspa alphabet. ("Seal script" is a style of writing, used for name seals and official stamps. Phagspa had a seal script variant modeled after the appearance of the Chinese seal script of its day. In this guise it was called the 蒙古篆字 Mongol Seal Script, with only the initial character distinguishing it from the 古篆字 credited by the "Hunmin Jeong-eum" as the source of hangul.) There were many Phagspa manuscripts in the Korean palace library, and several of Sejong's ministers knew the script well.