Liǎng jiā gōngsī de shìchǎng fèn'é cǐxiāo-bǐzhǎng.
The two companies' market shares move in opposite directions - one's loss is the other's gain.
Tips
usage
Use 此消彼长 when two quantities trade off against each other. Compare 此起彼伏, where two things rise and fall in alternation but not necessarily in opposition.
memory
Read literally: 此 this + 消 shrinks + 彼 that + 长 grows. A perfect see-saw.
