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》

Invite your partner on a date by sending her a plain brown envelope containing a tape.

假如要约对方，发一封信，里面放一盘磁带。

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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.

FORBES: U.S. Postal Service To Eliminate Saturday Delivery: Will It Save Tax Dollars?

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

FORBES: Magazine Article