Tone sandhi refers to tone manipulation rules governing the prounciation of tonal languages. Sandhi in Sanskrit means "putting together".
In Mandarin Chinese, the most common tone sandhi rule is that the leading syllable in a set of two third-tone syllables is raised to the second tone. For example, ni3hao3 (pinyin), the most common Mandarin greeting, is pronounced ni2hao3.
Languages with such rules include: