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英语语法上有所谓的“分布数”或“分配数”，其英文名称是 distributive number。

所谓的“分布数”或“分配数”，就是指在两组人或事物中，其中一组人或事物与另一组人或事物逐个匹配时所使用的“数”——用单数，就叫“分布单数或分配单数”；用复数，就叫“分布复数或分配复数”。

The students raised their hand.（分布单数或分配单数）

The students raised their hands. （分布复数或分配复数）

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分布复数用于复数名词短语之中，以表示一组实体与另一组实体逐个相匹配：

Have you all brought your cameras? [“Each has a camera]

Hand in your papers next Monday. [“Each has to hand in one paper.’’]

虽然分布复数是规范的用法，但是分布单数也可以用来强调各个个体。因此，我们常常在单数与复数之间可以有所选择：

The students raised their hand(s).

So children have understanding fathers / an understanding father.

We all have good appetites / a good appetite.

Pronouns agree with their antecedent(s).

Their noses need / nose needs to be wiped.

The exercise was not good for their back(s).

在成语或比喻中，有时必须用单数或最好用单数：

We are keeping an open mind. [*? open minds]

They vented their spleen on hint. [*their spleens]

They can’t put their finger on what’s wrong. [* their fingers]

有时，分布单数可以用来避免意思不明确：

Students were asked to name their favourite sport.

使用单数就清楚地表明只要说出一种运动即可。类似的例子有：

Children must be accompanied by a parent.