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When is a circle used at the start of a Hangul syllable block?
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In the Korean language writing system known as Hangul, the circle character (ㅇ) is used at the start of a syllable block when the syllable begins with a vowel sound.
Because every Hangul syllable block must begin with a consonant, the circle acts as a "null" or "silent" placeholder to indicate that the block starts with a vowel rather than a consonant sound. If a syllable begins with a vowel, the circle is placed in the initial consonant position to satisfy this structural requirement.