Four tones - Chinese Pronunciation Wiki

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Praat Manual: Tones in Mandarin – Corpus-based Mandarin Pronunciation Learning System

Romanization of Chinese - Wikipedia

Four tones - Chinese Pronunciation Wiki

Tones in Mandarin Chinese

Four tones - Chinese Pronunciation Wiki

Taiwanese Mandarin - Wikipedia

The Hacking Chinese guide to Mandarin tones, Hacking Chinese

Great start to #learnchinese tones with @chinesepod! Have instant audio playback.

Chinese Pronunciation Wiki

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Four tones - Chinese Pronunciation Wiki

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Alex Zorach's Tea Blog: Romanization of Chinese Tea Names: Wade-Giles vs. Pinyin

Obligatory and optional tone change rules in Mandarin, Hacking Chinese