distinguish up to five levels of pitch, though the Chori language of Nigeria is described as distinguishing six surface tone registers. Since tone contours may involve up to two shifts in pitch, there are theoretically 5 × 5 × 5 = 125 distinct tones for a language with five registers. However, the most that are actually used in a language is a tenth of that number. Several Kam–Sui languages of southern China have nine contrastive tones, including contour tones. For example, the Kam language has 9 tones: 3 more-or-less fixed tones (high, mid and low); 4 unidirectional tones (high and low rising, high and low falling); and 2 bidirectional tones (dipping and peaking). This assumes that checked syllables are not counted as having additional tones, as they traditionally are in China. For example, in the traditional reckoning, the Kam language has 15 tones, but 6 occur only in syllables closed with the voiceless stop consonants , or and the other 9 occur only in syllables not ending in one of these sounds. Preliminary work on the Wobe language (part of the Wee continuum) of Liberia and Côte d'Ivoire, the Ticuna language of the Amazon and the Chatino languages of southern Mexico suggests that some dialects may distinguish as many as fourteen tones or more. The Guere language, Dan language and Mano language of Liberia and Ivory Coast have around 10 tones, give or take. The Oto-Manguean languages of Mexico have a huge number of tones as well. The most complex tonal systems are actually found in Africa and the Americas, not east Asia. Tonal change Tone terracing Tones are realized as pitch only in a relative sense. "High tone" and "low tone" are only meaningful relative to the speaker's vocal range and in comparing one syllable to the next, rather than as a contrast of absolute pitch such as one finds in music. As a result, when one combines tone with sentence prosody, the absolute pitch of a high tone at the end of a prosodic unit may be lower than that of a low tone at the beginning of the unit, because of the universal tendency (in both tonal and non-tonal languages) for pitch to decrease with time in a process called downdrift. Tones may affect each other just as consonants and vowels do. In many register-tone languages, low tones may cause a downstep in following high or mid