In Chinese, it is noticed that when tones come together, certain allophonic and phonemic changes occur, known as tone sandhi. As to Mandarin, its tone sandhi is the simplest one and can be explained as tonal dissimilation.
In Fukien dialect, when syllables are members of compound word or syntactical unit, the last syllable retains its proper tone, but all the preceding syllables shift their tones with precise regularity according to the tonal law. This tonal law is phonemic in nature and may be simplified as follows:
陽平→陽去→陰去←/-q/—陽入
↑ ᅵ ↑ ᅵ ↓ /-p・-t・-k/
陰平← 陰上←/-q/—陰入
Tone sandhi illustrated above is obligatory and even in a sentence we can get several tonal groups, each dividing the sentence up into various syntactical units. It is evident that the tonal groups correspond to syntactical units. This tonal behaviour may be thought to have combining function of each syllable as a continuous whole and synthesizing function of each relating syntactical unit.