Letters for vowels were created from the consideration of not olny sounds but also philosophical principles. Vowel letters are also divided into basic letters and ones derived from them. People who created Han-geul saw three vowels. 'ㆍ', ‘ㅡ’ and ‘ㅣ’ as basic. The letter 'ㆍ' symbolizes the round shape of the sky, the letter ‘ㅡ’ means the plate shape of the land, and the letter ‘ㅣ’ indicates the upright shape of the human. These three, sky, land and human, are considered as the fundamental features of all things in Oriental philosophy.

Other vowels were made by properly combining those three letters. If 'ㆍ' is written over the ‘ㅡ’, ‘ㅗ’ is generated. If 'ㆍ' is put under the ‘ㅡ’, it becomes ‘ㅜ’. If 'ㆍ' is written on the right side of ‘ㅣ’, it becomes ‘ㅏ’, while 'ㅓ' is made by putting 'ㆍ' on the left side of 'ㅣ'. Even though today's ‘ㅗ’, ‘ㅜ’, 'ㅏ', and 'ㅓ' look as if horizontal and vertical lines are combined, at the time

 that Han-geul was created, it clearly showed that each letter came from the combination of 'ㆍ' and 'ㅡ', or 'ㆍ' and ‘ㅣ’. Each of ‘ㅛ’, ‘ㅠ’, ‘ㅑ’, ‘ㅕ’ has the same structure of each of ‘ㅗ’, ‘ㅜ’, ‘ㅏ’, ‘ㅓ’ except that 'ㆍ' is written twice. ‘ㅛ’, ‘ㅠ’, ‘ㅑ’, ‘ㅕ’ sound similar to ‘ㅗ’, ‘ㅜ’, ‘ㅏ’, ‘ㅓ' respectively, except that they are diphthongs(double vowels) with the glide(semi-vowel) 'ㅣ' attached in front. In this regard, the relationship between simple vowels and double vowels is reflected in the shapes of vowel letters.
Today, the letter 'ㆍ' is not used any more because the sound this letter used to represent disappeared now. Other letters for double vowels were made by combining letters for constituent simple vowels. For example, ‘ㅗ’ and ‘ㅏ’ combine to make ‘ㅘ’ and under the same rule, ‘ㅜ’ and ‘ㅓ’ combine to be ‘ㅝ’. Today, 'ㅐ' and 'ㅔ' are short vowels, but at the time when Han-geul was first made, they were double vowels sounding /aj/ and /ej/.

 Therefore, it was very reasonable to make 'ㅐ' by combining 'ㅏ'and 'ㅣ' and 'ㅔ' by combining 'ㅓ' and 'ㅣ'. The same rule is applied to ‘ㅚ’, ‘ㅟ’, ‘ㅒ’, ‘ㅖ’, ‘ㅙ’, ‘ㅞ’. The sounds which these letters represented were diphthongs(double vowles) or triphthongs(triple vowels) just like the way their shapes suggest. But, today they were simplified to simple or double vowels, so that the relationship between letters and sounds is not so clear any more.

Thanks to the systematicity of Han-geul, we can learn Han-geul very easily. Our very low illiteracy, which is internationally recognized, seems to be due to the scientific feature of Han-geul.

 The scientific feature of Han-geul also shows its value even in this Digital Era. For example, think about the input system of cellular phone used for sending text messages. Because the number of keys is limited, to each key are assigned more than one letter. In case of Roman Alphabet, the letters that belong to the same key have no common ground because shapes of letters have nothing to do with their sounds.

In contrary, in case of Han-geul, if the shapes of letters are similar, their sounds are also similar. Because of this, it is easy to assign several similar sounds/letters to one key. And so it is also easy to know which letters are on which key, so that users can input text message fast using cellular phones.