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Segments without letters

The blank cells on the summary IPA chart can be filled without much difficulty if the need arises.

The missing retroflex letters, namely ⟨ ᶑ ꞎ 𝼅 𝼈 𝼊 ⟩, are "implicit" in the alphabet, and the IPA supported their adoption into Unicode. Attested in the literature are the retroflex implosive ⟨ ᶑ ⟩, the voiceless retroflex lateral fricative ⟨ ꞎ ⟩, the retroflex lateral flap ⟨ 𝼈 ⟩ and the retroflex click ⟨ 𝼊 ⟩; the first is also mentioned in the IPA Handbook, and the lateral fricatives are provided for by the extIPA.

The epiglottal trill is arguably covered by the generally trilled epiglottal "fricatives" ⟨ ʜ ʢ ⟩. Ad hoc letters for near-close central vowels, ⟨ ᵻ ᵿ ⟩, are used in some descriptions of English, though those are specifically reduced vowels (forming a set with the IPA reduced vowels ⟨ ə ɐ ⟩), and the simple points in vowel space are easily transcribed with diacritics: ⟨ ɪ̈ ʊ̈ ⟩ or ⟨ ɨ̞ ʉ̞ ⟩. Diacritics are able to fill in most of the remainder of the charts. If a sound cannot be transcribed, an asterisk ⟨ * ⟩ may be used, either as a letter or as a diacritic (as in ⟨ k* ⟩ sometimes seen for the Korean "fortis" velar).