The idiom is one kind "the fixed word group", has the relatively fixed union way.

成语是一种“固定词组”，有着相对固定的结合方式。

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Temporary two-character word group and fixed two-character word group have no clear boundaries between groups, partly because the internal structure of word groups constitutes a form of consistency;

临时二字组和固定二字组之间没有明确的界限，一方面是由于字组内部结构构成方式的一致性；

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