The International Phonetic Alphabet (IPA) consists of more than 100 letters and diacritics. Before Unicode became widely available, several ASCII-based encoding systems of the IPA were proposed. The alphabet went through a large revision at the Kiel Convention of 1989, and the vowel symbols again in 1993. Systems devised before these revisions inevitably lack support of the additions they introduced. Only language-neutral systems are discussed below because language-dependent ones (such as ARPABET) do not allow for a systematic comparison.