Tone letters are letters that represent the tones of a language, most commonly in languages with contour tones. == Chao tone letters (IPA) == A series of iconic tone letters based on a musical staff was invented by Yuen Ren Chao and adopted into the International Phonetic Alphabet. Combinations of these tone letters are schematics of the pitch contour of a tone, mapping the pitch in the letter space and ending in a vertical bar. For example, [ma˨˩˦] represents the mid-dipping pitch contour of the Chinese word for horse, 马 mǎ. Single tone letters differentiate up to five pitch levels: ˥ 'extra...