For simplicities sake, I will use the following abbreviations:
- LV is any single-character Hangul syllable block composed of a leading consonant and a vowel, e.g. 모
- LVT is any single-character Hangul syllable block composed of a leading consonant, a vowel, and a trailing consonant, e.g. 목.
- L is a any single-character Hangul leading consonant, e.g. ᄆ
- V is any single-character Hangul vowel, e.g. ᅩ
- T is any single-character Hangul trailing consonant, e.g. ᆨ
Standards for encoding Hangul syllable blocks
There are two different standards for encoding Hangul syllable blocks:
- According to KS X 1026-1 (English translation: KS X 1026-1), a Hangul syllable block must be encoded as LV or LVT. If no Lv or Lvt is available, (that is, when using older letters), a complete single Hangul syllable block must be encoded by L V T? (a sequence of one L followed by one V followed zero or one T). So there are only three options: Lv, Lvt, L V T?
- According to UAX #29: Unicode Text Segmentation § Hangul Syllable Boundary Determination, a Hangul syllable block can contain sequences of L, V, or T, and there can be combinations of Lv or Lvt with either L, V or T. The possible ways to encode a single Hangul syllable block are thus the following: L+ V+ T* (a sequence of one or more L followed by one or more V followed by zero or more T), L* Lv V* T* (a sequence of zero or more L followed by one Lv followed by zero or more V followed by zero or more T), or L* Lvt T* (a sequence of zero or more L followed by one Lvt followed by zero or more T.
The Unicode standard has been widely adopted with regards to how applications identify what sequences behave like a single Korean syllable block, e.g. when selecting text or moving the cursor. KS X 1026-1, on the other hand, has apparently not even been adopted by Samsung Android, cf. Peter Constable’s second 2020-09-04 comment to “‘Ancient Hangul’ is an exaggeration”. With regards to fonts, there is probably no font yet that displays any sequence behond KS X 1026-1 as a single Korean syllable block. If such a font were written, the wide adoption of the Unicode standard would ensure that it behaves correctly in most applications.
Consonant or vowel sequences in the different standards
KS X 1026-1 does not allow sequences of consonants or vowels For instance, there can be no T T sequences like ᆨᆺ or Lvt T sequences like 목ᆺ.
Unicode allows sequences of consonants or vowels. T T sequences like ᆨᆺ or Lvt T sequences like 목ᆺ are perfectly acceptable and behave as if they formed single Hangul syllable blocks – there is just no font yet to display them as such.
The question is whether single Hangul characters such as ㅔ or ㄳ should be considered atomic letters or multigraphs composed of other Hangul characters, in this case, of ㅓ and ㅣ or of ㄱ and ㅅ respectively. There are a number of reasons to consider them composed multigraphs:
- The original account of the Hangul, the Hunminjeongeum, describes them as compositions.
- They look like compositions, with the individual elements being easily identifiable and sometimes not even touching.
- Their names are composed of the names of the individual elements.
- The individual elements often represent different phonemes, especially in the case of characters composed of two different consonant elements such as ㄳ.
- KS X 1026-1 recognizes them as complex letters.
- Common keyboard layouts allow typing the individual letters seperately.
On the other hand, those characters have no Unicode decomposition rules, canonical or compatibility, for being split into their individual elements. This cannot change because of Unicode’s Normalization Stability. However, there is a ticket accepted for Unicode’s Common Locale Data Repository to decompose those characters for the purpose of correct collation, cf. CLDR-10246 • Updated Hangul collation tailoring (probably a follow-up on L2/17-078).
So there are strong reasons to think of characters such as ㅔ or ㄳ as multigraphs.
Display of consonant or vowel sequences
Sequences of consonant or vowel characters are allowed by Unicode (though not by KS X 1026-1). According to the Unicode Text Segmenation rules, they should be displayed in a single syllable block. How should that look visually?
There are well-defined rules for the arrangement of Hangul letters into syllable block. Basically, those rules have been layed out in the Hunminjeongeum:
- Consonant sequences are arranged from left to right (except for ㅇ, which is placed below).
- Leading consonants signs are placed above the horizontal vowel signs or to the left of vertical vowel signs.
- Trailing consonants are placed below.
By these rules, the only way to display the sequence 목ᆺ in a single syllable block is to make it look like 몫.
Is there a difference between hangul digraphs or trigraphs and sequences of the respective single letters?
Visually, I think there cannot be any difference between Hangul multigraphs and the corresponding sequences of consonant or vowel letters – if those sequences are displayed in a single Hangul syllable block as per the Unicode Text Segmentation rules. So 몫, which contains the digraph ㄳ, will look like 목ᆺ, which contains a sequence of ㄱ and ㅅ.
By the logic of the Hangul script, I think there cannot be a difference either. The character 몫, containing the digraph ㄳ, can be analyzed as a sequence of ㅁㅗㄱㅅ – exactly like the sequence 목ᆺ.
The only way by which 몫 and 목ᆺ displayed as 몫 would remain different is the Unicode normalization. The character 몫 decomposes to ㅁㅗㄳ and normalizes back to itself. The sequence 목ᆺ decomposes to ㅁㅗㄱㅅ and also normalizes back to itself. This is a potential security problem with no easy answer.