According to 훈민정음 해례본, if you open your mouth while pronouncing ㆍ, it becomes a sound of 'ㅏ'(ㅏ與ㆍ同而口張) and if you close your mouth, it becomes a sound of ㅗ(ㅗ與ㆍ同而口蹙). It is assumed that when Hunminjeongeum was created, ㆍ was the sound of [ʌ], somewhere between ㅏand ㅗ. Between the 18th and 19th centuries, however, there was a phenomenon in which the ㆍ's rhymes suddenly changed intoㅏ(usually after the first syllable) and ㅡ (usually after the second syllable). See this example: ᄯᆞᄅᆞᆷ → 따름. ᄲᆞᄅᆞ다 → 빠르다. This change of sound caused a rapid collapse of the Korean language's Holsoiri eowulrim(홀소리어울림, vowel harmony), which was observed even in the Middle Ages. This conflict between the positive vowels and the corresponding negative vowels began to break down and has affected the overall vowel harmony of Korean words, which continues to the present day.