IPA numbers are a legacy system of coding the symbols of the International Phonetic Alphabet.
Following the Kiel Convention in 1989, most letters, diacritics and other symbols of the IPA were assigned a 3-digit numerical code, with updates through 2005.
The purpose was to identify IPA symbols explicitly in an era of competing computer encodings, and thus to prevent confusion between similar characters (such as ⟨ɵ⟩ and ⟨θ⟩, ⟨ɤ⟩ and ⟨ɣ⟩, ⟨ʃ⟩ and ⟨ʄ⟩, ⟨ɫ⟩ and ⟨ɬ⟩ or ⟨ǁ⟩ and ⟨‖⟩) in such situations as the printing of manuscripts.
A few superscript IPA letters, including a single vowel, have also been assigned numbers in this range.
In addition, there is the pre-composed character ⟨ɫ⟩, the loop-tail ⟨g⟩ (see the main consonant chart above), and the implicit IPA letter ⟨ᶑ⟩.
⟨ı⟩, which was once needed to create ⟨i⟩ with a diacritic, two letters retired at Kiel, ⟨ɩ ɷ⟩, and the three non-rhotic vowels added in the 1993 and 1996 updates to the IPA, ⟨ɘ ɞ ʚ⟩, were numbered downward from 399.