Definition
In Korea, the design principles of Hunminjeongeum are the scientific rules King Sejong the Great used in 1443 to create the 28 original letters of Hangul (then called Hunminjeongeum, "the correct sounds for instructing the people"). He modeled basic letters on the shapes of the speech organs and on the shapes of heaven, earth, and humans, then added strokes or combined letters to build the whole system step by step.
The Birth of Consonants: Drawing the Shape of the Mouth
What if you could see how your tongue and lips move when you talk? King Sejong studied the organs that make speech sounds as closely as an X-ray would. From that, he created five basic consonant letters: ㄱ (g), ㄴ (n), ㅁ (m), ㅅ (s), and ㅇ (a silent or "ng" sound).
ㄱ came from the root of the tongue blocking the throat. ㄴ came from the tip of the tongue touching the upper gum. ㅁ came from the shape of the lips. ㅅ came from the shape of the teeth. ㅇ came from the shape of the throat. This is called the principle of pictographic imitation of the speech organs.
Then he added one stroke at a time as the sound got stronger. Adding a stroke to ㄱ gives ㅋ. Adding a stroke to ㄴ gives ㄷ and then ㅌ. This is called the "stroke-adding principle." Just by looking at a letter, you can feel the sound getting stronger.
The Birth of Vowels: Basic Symbols for Heaven, Earth, and Humans
Vowels were built from the basic elements of nature and the universe. Everything starts with just three symbols: a dot (·, called "arae-a") for the round sky, a horizontal line (ㅡ) for the flat earth, and a vertical line (ㅣ) for a person standing upright.
Combine these three and you get the vowels we use today. Sky (·) meets earth (ㅡ) to make ㅗ and ㅜ. Person (ㅣ) meets sky (·) to make ㅏ and ㅓ. Add a second dot and you get ㅛ, ㅠ, ㅑ, and ㅕ, the vowels with a "y" sound added.
The system was designed so that a complicated set of vowels could be assembled from just three basic parts. Once you know the logic, you can read and write every vowel with ease.
To Be More Precise: A Featural Writing System
To be more precise, Hunminjeongeum is more than a set of picture-letters. It is sometimes classified as a featural writing system (one whose letter shapes reflect features of sounds). British linguist Geoffrey Sampson classified Hangul this way in 1985, though some scholars question that label.
For example, the English letters K and G stand for sounds made in nearly the same place in the mouth, but the letters share nothing in shape. In Hangul, ㄱ and ㅋ look alike. So just from the shapes, you can tell they are made in the same spot and that ㅋ is the stronger sound.
Hunminjeongeum also had "irregular letters" (ㆁ, ㄹ, ㅿ) that don't follow the stroke-adding principle. The Haerye (the original explanatory text, known as the Hunminjeongeum Haeryebon) explains that these letters look different, but the added-stroke meaning (a stronger sound) doesn't apply to them. It separated them on its own phonological grounds. Of these, ㄹ is still used today, while ㆁ and ㅿ are no longer used.
Common misconceptions
King Sejong got the idea for Hangul by chance from the lattice patterns of a traditional hanok window.
As the Hunminjeongeum Haeryebon clearly records, it was a carefully engineered writing system, based on close study of the shapes of the speech organs and the sound system of the language.
The vowel letters are just sticks and dots put together at random.
They were built on a worldview of heaven (·), earth (ㅡ), and humans (ㅣ), and they precisely apply a sound rule: the bright and dark quality of vowels (yang and yin vowels).
Where you meet it
Add a stroke to ㄴ to get ㄷ, then add another to get ㅌ. The letter shapes show the sound getting stronger, step by step.
Take the basic vowels ㅡ and ㅣ and attach the sky dot (·) above, below, left, or right. This systematically produces ㅗ, ㅜ, ㅓ, and ㅏ.
In one sentence
Consonants modeled on the speech organs and vowels modeled on heaven, earth, and humans were extended by adding strokes and combining letters. The result is a design system that can write the sounds of Korean in an orderly way.