Hangul
In this book, the design ideas of Hangeul and information about its use were written down in detail. The book Hunminjeong'eum Haeryebon states the two design ideas of Hangeul. The first one is that the consonants of Hangeul look like vocal organs. The second one is that the vowels of Hangeul are based on Cheonji'in (천지인). Cheonji'in refers to the three elements of philosophy which are sky (천), earth (지) and human (인). Because of these ideas, Hangeul is well known for being easy to learn.