information:highlighting, structural:reformatting
Morpho-syllabic blocks

Except for a few grammatical morphemes prior to the twentieth century, no letter stands alone to represent elements of the Korean language. Instead, letters are grouped into syllabic or morphemic blocks of at least two and often three: a consonant or a doubled consonant called the initial (초성, 初聲 choseong syllable onset), a vowel or diphthong called the medial (중성, 中聲 jungseong syllable nucleus), and, optionally, a consonant or consonant cluster at the end of the syllable, called the final (종성, 終聲 jongseong syllable coda). When a syllable has no actual initial consonant, the null initial ㅇ ieung is used as a placeholder. (In the modern Korean alphabet, placeholders are not used for the final position.) Thus, a block contains a minimum of two letters, an initial and a medial. Although the Korean alphabet had historically been organized into syllables, in the modern orthography it is first organized into morphemes, and only secondarily into syllables within those morphemes, with the exception that single-consonant morphemes may not be written alone.

The sets of initial and final consonants are not the same. For instance, ㅇ ng only occurs in final position, while the doubled letters that can occur in final position are limited to ㅆ ss and ㄲ kk.

Not including obsolete letters, 11,172 blocks are possible in the Korean alphabet.