He placed the paper in a plain envelope.

他把论文放在一个空白信封里。

《柯林斯英汉双解大词典》

And I remain impressed that for less than the price of a cup of coffee, I can pop a plain envelope into a box and have confidence, more often than not, that it will be delivered halfway around the world.

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"All you get is a plain brown envelope, " says Jeffrey Kindler, Pfizer's vice chairman.

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