Zero consonant
In orthography, a zero consonant, silent initial, or null-onset letter is a consonant letter that does not correspond to a consonant sound, but is required when a word or syllable starts with a vowel (i.e. has a null onset). Some abjads, abugidas, and alphabets have zero consonants, generally because they have an orthographic rule that all syllables must begin with a consonant letter, whereas the language they transcribe allows syllables to start with a vowel. In a few cases, such as Pahawh Hmong below, the lack of a consonant letter represents a specific consonant sound, so the lack of a consonant sound requires a distinct letter to disambiguate.
Uses
The letter א aleph is a zero consonant in Ashkenazi Hebrew. It originally represented a glottal stop, a value it retains in other Hebrew dialects and in formal Israeli Hebrew.
In Arabic, the related letter ا alif is often a placeholder for a vowel.
In Korean hangul, the zero consonant is ㅇ ieung. It appears twice in 아음 a-eum "velar consonant". ㅇ also represents ng at the end of a syllable, but historically this was a distinct letter.