those of the languages of Liberia and southern China. The Chao tone letters have two variants. The left-stem letters, , are used for tone sandhi. These are especially important for the Min Chinese languages. For example, a word may be pronounced in isolation, but in a compound the tone will shift to . This can be notated morphophonemically as , where the back-to-front tone letters simultaneously show the underlying tone and the value in this word. Using the local (and internationally ambiguous) non-IPA numbering system, the compound may be written . Left-stem letters may also be combined to form contour tones. The second Chao letter variant are the dotted tone letters , which are used to indicate the pitch of neutral tones. These are phonemically null, and may be indicated with the digit '0' in a numbering system, but take specific pitches depending on the preceding phonemic tone. When combined with tone sandhi, the left-stem dotted tone letters are seen. An IPA/Chao tone letter will rarely be composed of more than three elements (which are sufficient for peaking and dipping tones). Occasionally, however, peaking–dipping and dipping–peaking tones, which require four elements – or even double-peaking and double-dipping tones, which require five – are encountered. This is usually only the case when prosody is superposed on lexical or grammatical tone, but a good computer font will allow an indefinite number of tone letters to be concatenated. The IPA diacritics placed over vowels and other letters have not been extended to this level of complexity. Africa In African linguistics (as well as in many African orthographies), a set of diacritics is usual to mark tone. The most common are a subset of the International Phonetic Alphabet: Minor variations are common. In many three-tone languages, it is usual to mark high and low tone as indicated above but to omit marking of the mid tone: má (high), ma (mid), mà (low). Similarly, in two-tone languages, only one tone may be marked explicitly, usually the less common or more 'marked' tone (see markedness). When digits are used, typically 1 is high and 5 is low, except in Omotic languages, where 1 is low and 5 or 6 is high. In languages with just two tones, 1 may be high and 2 low, etc. Asia In the Chinese tradition, digits are assigned to various tones (see tone number). For instance, Standard Mandarin Chinese, the official