Hangul

Hangul drew its consonants from the shape the mouth makes and its vowels from sky, earth, and human being. Add a stroke and the sound hardens. Twenty-eight letters cover that many sounds not because more letters were added, but because a rule for combining them was designed.

첫째 단서 — 구조The consonants came from the shape the mouth makes to produce a sound; the vowels from sky, earth, and human being. Add a stroke and the sound hardens. One stroke turns ㄱ into ㅋ.
둘째 단서 — 목적The preface says it was made out of pity for people who could not put their meaning into writing. Easy to learn was the design condition. It is a rare case where the purpose survives as a document.
셋째 단서 — 질서Twenty-eight letters carry that many sounds. Few letters covering a wide range came not from adding more letters, but from designing a rule for combining them.

The shape inside a mouth became a letter. A design that made sound visible.